Nothing realy exept licencing

Maybe consider using forgejo (gitea fork used by codeberg)

And i do think, some changes would be needed but nothing big, also it wouldnt ne activated by default.

I second matrix. Its also federated and has mutible clients. But like lemmy under development (but more advanced in the development process)

Obtainium ist a packetmanager pulling updates/new versions from the source (i.e. some form of git)

And the three apps are Lemmy/kbin clients (under heavy development) where some are not available at more conventional packagemanagers like F-Droid.