I'm having trouble understanding your math here, and why this is such a big issue at your income/saving level, stage of life, and level of professional flexibility.
Since you hate work so much, but have pretty good flexibility about hours, etc., have you considered cutting back your hours NOW so that you can have a better quality of life? Especially when your kids are little, there is so much stress (and I never had two under age 3 -- DS was 3y 8 mo when DD was born). Yes, reducing your hours will reduce your immediate savings, but maybe it will increase your quality of life so much that working longer won't seem so awful.
Alternatively, is there work you could do -- perhaps as a highly paid consultant -- that would take advantage of your training in dentistry while allowing you to get away from the practice OF dentistry, since you don't seem to enjoy it. R&D? Sales? Insurance consulting?
I know you mentioned your wife is unwilling to move away from family, so I understand that may limit your options. But it just seems so sad that this is such a black and white issue for you. I understand your wife's desire to try for a girl. I was lucky -- I got one of each and then could happily quit, and I think our family situation is perfect (my kids are awesome, too!) But I do wonder if I would have wanted a 3rd if DD had been a boy.
I'm not trying to say that your wife is totally right here -- but at the same time it just makes me sad that you can't find a way to meet both your needs/desires. I understand she mentioned the possible inheritance when you raised the idea of her working more -- I don't think that is the way to go, as you never know what will happen down the road, and both or either sets of parents could lose all their money to a medical crisis or some other disaster.
That being said, another option, if you are able to talk openly with them about financial matters is to talk with them about your dilemma -- not in a money-grubbing way, but just a "hey, wife would really like to have another kid. At the same time, we feel fully funding college for all our kids is important, but we both want to be able to cut back at work in 5 years or so. Not sure what to do." Maybe one or both sets of grandparents already have plans to set up a 529 or trust for the grandkids -- that kind of generation skipping wealth transfer is not uncommon among people with means. Gifting the money earlier means the heirs actually take advantage of it while the benefactors are alive to see it. I hope to be in the position to do that for my grandkids. It sounds like both sets of parents were supportive of you guys pursuing education -- is there any reason why they wouldn't want the same for their grandkids?