I know of nothing more than what people mentioned in this thread, sorry. :) Except maybe… Have you considered writing? If what you’re after is telling stories rather than coding, it’s the easiest and yet most powerful way to explore your imagination, what you can do in a videogame is ridiculously limited compared to what you can do in a written story. And it’s free. :)
It will definitely fall into the category of “part of a larger collaboration ecosystem”, but Gitlab checks all your boxes : it’s open-source, installable on premise, allows groups, with their own discussions, handling uploads… And then, it can also host git repositories. 😅 They made an insane work on workflows so you can pretty much customize the tool how you want, including executing automated scripts when whatever event happens.