@Youlookgorge sending you love & support while you try to figure this out!!! Well done for toughing it out through the comments section, it's brutal, but in fairness it does cover more or less the whole range. Only you know whether it's possible to pick a path through it as a couple, or whether separating is likely to be the best option for you & DC.
The sympathy I have for your DH circumstances:
• parenting under 5s is repetitive and relentless and dull
• as an ND adult, this is likely to drag your mental wellbeing down harder and faster than an NT parent doing the same relentless parenting work
• as a founder who has cashed in, there is a parallel and well-documented issue around the 'what do I do now/how do I find my way/my value in life' which also really weighs on mental wellbeing
The reality checks:
• if you don't engage with the hourly requirements of DC, regardless of how unrewarding it is, you are either neglecting their development or wellbeing, or storing up trouble for poorly adjusted children at a later age (or both!)
• housework and feeding a family of 4 is also unrewarding, but there is a minimum acceptable threshold to remain hygienic and with enough nutrients in you all to function effectively and fend off illness as each round of nursery lurgy inevitably arrives
• finding your 'why' will be a key motivator for the next phase of your life - it sounds like he has some well-developed thoughts on this....other than that he's treated it as a him project, not a household/family project.
If your overwhelming sense at the moment is that you love each other enormously and you want to continue building your future together, I'm sure you can find more positive steps to make this path work for you.
If your sense is that he's checked out & it's irretrievable, figure out your exit route now. I would suggest that positioning his (business) exit fund as a pot in 3 parts would be a good idea - one third for each of his dependents. That's him, and each DC for the next 25 years. You are going out to work & supporting yourself (as well as doing the lion's share of the household support) and are not a dependent. But in the event of a divorce, sod 50:50 - he's got 2 children to support, so he can carve up his sellout from business 1 three ways and use his third to fund uni/whatever comes next. If he's checking out of all meaningful involvement, the kids should have their 1/3 lump each now to cover the rest of their upbringing. At that point get the nanny, and put all your effort into a stable household that DC can thrive in, around whatever hours DH decides he is actually prepared to engage each month.
If you can carry on together, I would suggest positioning that list of reality checks above as projects to solve - try to draw his ND focus onto the project of 'our children' and 'our functional living arrangements' - what do they need in their daily upbringing to thrive at each age?' What do you have to cover that is non-negotiable to function as a household? How can he navigate his next career steps without it being a net drain on the 3 other people he should care most about in his life.
If you want to go this path, couples therapy would be well worthwhile, particularly if you've never done any before. If you can find someone that is used to working with ND partnerships even better, bonus points if they've helped founder/exiters over their 'what next' hurdle before. If you go to someone vanilla, with no experience of these conditions or circumstances, you risk getting advice that is as generic or NT focused as some of the comments on here.
Only you know whether your foundation with DH is strong enough to fight for a future together, and how hard it will be to engage him to think about 4 of you in his daily & long-term decision making, rather than just himself.
We've made it work in our house because our love for each other is enormous AND I wanted to put our DC at the centre of all our decision making. But as the ND one, I nearly sank by the time the second one was 5 years old in the process...
Good luck, whichever path you end up on 