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?

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^^

a simple file system will do that?