Needless to say, you can’t really plan for everything to go perfectly. I would always say, “I can’t think of a better time to quit your job than when you just got married and have a baby on the way.”
On another note, I was putting in a ton of hours toward creating my own thing ( https://www.facebook.com/mediamoonllc/ – https://media-moon.com/ ) for years while I wasn’t at the 9-5 (and sometimes while I was at the 9-5 (mainly lunchtime)). Many of the vacation days or time off from the 9-5 were days spent on my side thing.
If at all possible, get as much going while you still have the day job in order to lighten the blow when you switch over to your own gig. Saving money for the transition is one thing, but if your side gig is bringing enough money in, hopefully there’s enough to cover a decent “margin of error” (i.e. if there’s an emergency that causes a financial headache).