OP,
Why you had more than one child with a man you are profoundly unhappy is not why you started this thread, so I will try to address your concerns/questions with the information you have provided. My advice is based solely on information you provided in this thread, and may fail to address issues not mentioned here. My apologies if my reply is off the mark.
Is there no way that you can tell your husband that you want a divorce, and work out the details together? Many couples decide to divorce, and work together to sort out living arrangements, visitation, finances, etc. before divorce papers are even filled. Do you have a room in your home that you, or he, could use until moving day?
Navigating a divorce can be tricky in the ideal situation, which yours is not. Based on your responses you are not in fear of harm from your STBX husband, and you have the financial means to support your children, and yourself. Your situation, though not ideal, did not seem to be dire. Thankfully. I am sorry that your marriage has become intolerable, but thankfully you have the means to leave, and provide your children a safe, looking home. Stability will take time, but it is possible.
With that in mind, have you thought about childcare since you will very likely have to work full time to support your family? If you don't already have childcare, and plan on attending it, be picky. I realize that finances often dictate what is available, but a safe, nurturing environment, is priceless. As a mother I happily gave up life's little luxuries to ensure that my children were cared for by someone who helped them thrive. If you do not have childcare ahead thought out, start researching now. Ask friends/family for recommendations. Meet with potential providers, collect references and contact them, make sure they are correctly licenced or insured if the law requires them to be, etc. This is one area that you should demand excellence in the service provided. Childcare is likely to be one of your largest expenses. Arranging for a caregiver now will help to ensure that you are awarded adequate child support so that it can be provided.
Will you be moving, or do you intend on keeping your current home? If you plan on moving, have you been looking for places to live? I would suggest trying to keep your school-aged children in their current school if at all possible. Divorces are emotionally very difficult on children, so having people they know in their support system would be very beneficial.
If you plan on leaving in August, it may help to get a storage unit and start acquiring furniture and other household items that you know you will need in your new home. It's much easier to do so over a little at a time to help lessen the financial impact. Keep an eye out for good deals. It may help the children adjust if you allow them to help decorate their new bedrooms. Divorces often leave a child feeling lost and powerless. Letting them help decorate may help them to feel like they have a voice, and some control in what is happening in their life.
Talk to your friends and family about your intention to divorce so they can help support you emotionally. With a new baby coming you are very likely to feel overwhelmed, at the very least. Giving your support network a heads up about your situation will allow them time to decide how/when they can help.
You might have a friend or family member who is willing to babysit, help clean, help set up your new home, or help you move/pack from your marital home, etc. but they need advance notice so that they can take time off of work, or whatever they need to do. Springing your situation on your support system may make it difficult for them to be there for you. Talking to someone now, if you aren't already, will make the coming months less difficult to get through. Getting a divorce is difficult enough, without trying to do it all alone. Do you have items you need to sell, and a friend who is a master of selling? It might be a good way to make some money. It could also help get rid of items you do not want or need, and save you the trouble of moving it into your new home.
If you tell your husband your intentions, and he suddenly becomes the family-man- of-the-year, don't trust it. Continue with your plans, and if he wants to salvage your family, he can absolutely do so while living apart. There is nothing wrong with living separately as you work on your marriage. Many couples have had their relationship's saved because they put in the effort, and fell in love with their each other again. There is truth in the saying, "You don't know what you've got, until it's gone."
You and your STBX husband need to decide how to tell the children about the divorce. You also need to consider (and understand that some are subject to change, and more are likely to be added) things like visitation, "rules" regarding dating/new partners IE. No sleepovers while children are present until they meet the children. No revolving bedroom door while children are present. No new partner allowed to disciple children. (Address one another's reasonable concerns, whatever they may be). You may need to sell outside assistance, and should do so if you do.
Do not exclude your children from everything to try to protect them. Keep them informed in an age appropriate, information appropriate, way. Again, seek outside help if you need to; church, friend or family, therapist, their teachers, etc. Encourage them to talk to you about how they're feeling, and listen to their concerns even if they seem still or understand. It's a scary and confusing time for them, be there when they need you to be. Make sure that you and their father will tell them the same thing when asked why you're divorcing (if they ask, and they very likely will). Pay attention to their behaviors, and if they are acting out, emotional, or behaving in a new way, don't ignore it. Do not parent out of guilt by buying them things, or allowing misbehavior. While you may be getting a divorce, you are still the same mother.
There is no way to protect your children from all of the pain that the divorce will likely cause. There are numerous ways that you can make things as tolerable as possible. No matter what happens, never forget that children are NOT pawns to be used. Your children are THE priority in your situation. Everything you do throughout this process needs to be done with their best interests in mind. From where you live to the visitation arrangements, your childrens well-being is vital.
Again, if at all possible, you should talk to your husband about the end of your marriage. He may be as unhappy as you are, and feel grateful that the marriage is coming to an end. It is easy for people online to say, "Tell him!" But, we do not know how he will react, and you do. Do you think that his reaction will be negative? Will he drain the bank accounts and hide any assets you may have? Will he react with screaming, threats, excessive anger? Only you have any real idea how he will take the news.
Right now, either of you could empty your accounts. If you file for a divorce, typically the court will consider all assets (real property, cars, collectibles, etc.) life insurance policies, credit cards, checking/savings accounts, and/or contacts as "binding" until the courts ruling. What that means is that no changes can be made to them that can be considered detrimental to one's spouse, especially without their knowledge or consent. If one of you emptied out there bank account, or maxed out a credit card, it could (depending on your state laws) be considered Contempt if Court; subject to fines, and possible jail time, or both.
So, to sum everything up:
* Talk to your husband if you can.
If not, find a lawyer and file for divorce.
The petition to divorce can be withdrawn at any time by you. Do not wait to file. If you wait, and he finds out and wants to be petty, legally he is not prevented from draining all of your joint accounts or machine or joint credit cards. You may be financially liable regardless of if you were aware of his actions or not.
If you file for a divorce, and he drains your accounts with the intent on punishing you, have your lawyer contact the Court. It is unlikely that you will financially liable for his actions. (Depending on your state's law's. Your lawyer can explain everything to you.)
Life insurance policies, health insurance, or other policies cannot be cancelled by either party, or changed. (Again, it can vary by state. You can Google what will happen in the filling of a divorce petition in your state).
* If you file you may feel the need to move sooner. Your home life may become intolerable, forcing you to move. This may land you in a home or neighborhood that is less than desirable. Or, in a home that doesn't meet your needs, is in the wrong area for your kids to continue in their school, or is unaffordable.
* Waiting and saving money is a gray area. You are not in a volatile situation, or in fear of your or your childrens safety. Any money you have typically must be included during disclosure (in your divorce petition) and is not excluded just because you were trying to save money so that you could move. You can be held liable for any money that you have hidden. Telling the Court the money was a gift from a friend, or family member, is potentially problematic. If your STBX (or his lawyer, or the courts) proves that you lied, you will very likely be found in Contempt of Court.
* Finding childcare now keeps you from settling for a subpar situation when you finally do move.
* With a new baby on the way, spreading the move out over several months allows you find the best living situation possible for you and the children. Rushed or time-critical decisions may not be in the best interests of your finances, etc.
* You've been unhappy for years. Your husband had been aware of the marital problems for years. To your children, everything is happening NOW. They are not likely to understand that your decision to divorce had happened over time. Their entire world is changing. You understand it, they get likely don't. Regardless of how unhappy you are, they will need time to process what you think had been coming for years. Your first priority is to be their mother. Hold their hands and as gently as possible walk them through the divorce, and they will get through it (as best as possible). I'm sure that you will do the best that you can for them. Your posts do present you as a loving mother. Please, don't forget that, even if the divorce gets petty.
* Talk to a lawyer. It is in your best interest to be as prepared as possible. Find out if you are allowed to save money, without disclosing it. Perhaps your lawyer could give you an estimate of how much financial support your husband would provide.
Hopefully somewhere in my novel-length reply I have provided you with helpful information. I realize much of the common sense advice you likely already know. You are welcome to heed the advice in whatever way works for you. Or, ignore it all if you choose to .😉
I wish you the best of luck.