You should sit with your other co-founder(s) and present your current predicament just as you've done here. Your subsequent conversation should be weighing the cost (to the startup) of you leaving to go back to back to big tech vs. you staying and cutting back on your time commitment/responsibilities to where you find some balance. Hopefully, you all have a good enough relationship where you can be open, direct and h…
This seems like a textbook "good answer" from the book "Difficult Conversations". Don't try and figure out how people will think in the isolation of your own mind. To quote Wittgenstein: look and see! Or in this case, ask and see!
You may find your co-founders say, "We have a good runway. Let's increase your salary temporarily / issue you a bonus to help offset the increased expenditures. Is there a way for us to adjust our hours to better accommodate your needs? How do we take more off your plate so you can focus on your family, and only work on the most important things for the business with the time remaining?"
FWIW- This is also why finding the right co-founders matters a lot. Having deep trust with your co-founders so that you can have these types of difficult conversations matters a lot.