Regarding college expenses, he agreed to pay half of tuition in the divorce decree with no qualification on where they went. His wife, in an action to further increase child support, also asked the judge to make him pay pro rata instead of half, but the judge said no. He thought half was a good deal at the time, since half would be less than pro rata (although with child support factored in this year, it's probably equal to and possibly more than pro rata). Plus even if that weren't in the divorce decree, he's not willing to tell his kids they can't go to a certain school or they have to take out loans because they are too expensive (note that both of the oldest two qualified for a full ride at a really good public state school, but instead chose to go private and the parents have to pay full freight). Also, in his state he has to pay shared expenses until the kids graduate college (think cell phone bills, insurance, travel, etc).
Formerplayer, thank you for taking the time to write a thoughtful response. Your point about communication is something that's on my mind and I know I need to work on it.
Regarding over withholding 2019 taxes, I should clarify that I'm doing that partly because he is not making any payments on 2019 taxes right now. Instead, we're paying 2018 taxes from his monthly income (he's a partner and responsible to do his own quarterly payments), and until that amount is below $100,000, we will not qualify for a payment plan for 2018 (and note that I already took out a $100,000 HELOC in only my name because he has poor credit to cover the remainder of 2017 taxes). I anticipate we'll get our 2018 obligation down to $100,000 by April or May and then apply for a payment plan. Still, I probably should withhold from less from 2019 and allocate more to 2018 taxes. My reasoning, which may be flawed, is this: if our marriage falls apart, I'm very sensitive to what my obligation is to the IRS. It's still not too late to file 2018 or 2019 separately. I get that in a division of assets (or in our case debt) this will be offset, but as between me and the IRS, I'd be protected. I already had a wage garnishment scare with our 2017 taxes and a threatened tax lien from 2016 (both of which are now paid in full). Theoretically he could cover the tax liability from his IRA (I guess I could as well), but that would cost quite a bit in additional taxes and penalties.
Also, part of why I struggle to see the debt as "our debt" is because it is imposed on me without my consent. For example, for Christmas he and his ex wife can choose to buy the kids brand new iphones, computers, Garmin watches, skis, etc., and of course in my mind the money spent on those items is not being spent to pay taxes. That stuff is actually small in our big picture, but a source of resentment. Then there's sending them to a college that costs over $70,000 a year all in for the student, plus all his travel back and forth to see them at their various schools, which is not at all insignificant in our big picture. And what if there is no end to this, because he decides without my consent to buy his adult children cars, pay for graduate school, help them buy a house, pay for super expensive weddings, etc. It's this reasoning that makes me want to prioritize taking care of myself and baby with my income. But maybe it is delusional to think I have any control left.
Moving is a pretty good idea. It would free up enough equity to pay off our tax liability, but the monthly mortgage payments are pretty low, so from a monthly payment perspective, if we were to rent something modest but adequate, rent would probably not be much less than our currently mortgage payment (maybe as much as $1,000/month less). Plus he's not too keen on downsizing since he's older and prouder.
All this to say, maybe a financial divorce and a splitting of expenses is the way to go. He'll probably have a huge equity event the following year if I do that, but that might have never mattered anyway for reasons described above related to lack of control.