JabRef with some kind of cloud file sharing or sync? You don’t get a reference manager with a server component here, but maybe it could work. You don’t edit the bibtex file that often, so conflicts should be minimal.

Zotero is selfhostable, isn’t it?

a simple file system will do that?

How is it going to generate the proper source List? How am i going to automaticly Download a study and get its metadata with a simple file system? You don‘t seem to know the usecase of those programs^^