I did this for my first Steam game, which let me go full-time back in 2015 as an indie game developer and where I still am today. [1]

I'd get up early and get a few hours in before my more ordinary programming work as a Python backend developer at a fintech startup. And then on Saturdays I'd get something more like a normal days work done on the project.

It worked, and I'm glad I did it, but it definitely was a temporary situation, I don't think it's something you can do long-term without facing burnout or neglecting other important areas of life like family, friends, exercise, reading books, etc. So that last paragraph of the original post - making it sustainable - I'm glad to see it in there.

[1] The Cat Machine - https://store.steampowered.com/app/386900/The_Cat_Machine/