I can only speak to my own experience. This is how I managed being a new father, running a startup, having a side gig, and actively engaging in hobbies, all at the same time: Extreme time management, with schedules for everything, and those schedules were adhered to rigorously.

I was genuinely shocked at how much I could accomplish every day. I didn't keep that pace up forever, although I do continue with several of the practices I developed then and my current schedule is pretty full. Now when I look back on that time, I have a hard time believing that what I did was even possible. It didn't come without a cost, of course, and one of those costs was my marriage. These days I'm simply not willing to sacrifice so much.