You're absolutely right. I'd love to have some kind of income to hire him/someone else/a team to work on it full-time because I was always one of his three or so projects and we were both working for free and he was a full-time developer so money needed to come in at some point. I'd love to slip into a director/producer role and hand the work off to some really talented people, guiding them as they go because I think they'll get it. But continuing to work on it as a hobby for an hour or two a week is just a tease, especially when I have other creative things I'm working on and a full-time job.
I am "lucky" that due to the pandemic, I can work from home, & have off unpaid day (currently 4 days week), allowing me to work in the evening as well as off days.
So I get paid in my day job, assuring stable income, & I am less stressed working on my game. At least not from $.
I think it is important to have a schedule, follow it, as well as post regular updates, even if it's just for yourself. I post almost daily, even if its not much on twitter & often here as well, that it becomes a routine & I can motivate myself.
Also, I think part-time solo Game development is about giving up other stuff you enjoy. There is no 2 ways about it. Just what are you willing to give up, which not.
There are tonnes of books, games, TV shows I wanted to watch, but I just don't have time for. I wanted to have a youtube channel on gaming (level design & such) but I just can't do it.