Problem is that you cannot do anything significant or at scale on your own. Writing POC or some fun code on the weekends isn't satisfying as collaborating with group of people and accomplishing something worthwhile. Sure you can find an OS project but that requires significant time investment to make any major contributions and you likely have the same frustrations with it that made your original job un-fun.
As someone that's working full time on their own thing - I agree it is extremely difficult to do something major part time (let alone full time!!) When it comes to coding.
So I'd flip the script here and ask: does it need to be significant?
If the aim is to do something enjoyable, then maybe cut yourself some slack. Find joy in learning, or joy in helping a cause. Don't worry if you feel it's small. Do it for you. Also, say no to things that overextend you or divert you from your mission of enjoying your hobby.
Having a team lets you validate your ideas, inspire new ideas and motivate you when you to keep the project going when there are setbacks.