So it looks like the choice is made: Git.

I looked into Collab, and it is indeed a simplified, streamlined, limited version of Git, so I'll just stick with my good old Gitlab setup. I'm already enjoying that, and I think it'll provide all that I need.

Music

This was a big thing in the games that MEWS is inspired by, so I'll eventually have to figure out a solution to that. Music is pretty much the only thing I don't have a good enough handle on to really make viable in the game. I might try, using LMMS or MPT, which would be cool and fun, but I'd need some serious music theory lessons for that to work out well, I think. On the upside, MPT files are small and work well in Unity projects. I just don't know if I can convince possible contractors to use it.