How to get a part-time job?

start full time, than ask your boss. At least down to 4 days/week is often easy that way.

so far the only easy career-friendly option I found.

Maybe freelancing can work, but that's something else all together. Maybe you can find a startup with limited funds.

If you need some encouragement: https://codewithoutrules.com/2019/05/09/part-time-software-d... (also pushing his 50 bucks book about the topic, but maybe it's even worth it, idk...)