I have three kids and I still go to the gym, work and start side hustles. My kids went to nursery at 3.

It doesn't have to be as bad as you describe it, you need to communicate with the mom and agree on responsibilities. If you're providing and need time to keep up to date (so that you can have a career progression and provide even more), make it clear.

If you're both working, I doubt it will work. You're either going to hate each other, cannibalise both careers or not dedicate enough time to the kids.

After 3 it's easier as nursery / school give you some extra alone time - at that point I'd consider switching back to two workers in the family.