I know you said “no nextcloud”, but I often see posts from people who say nextcloud was overkill for them, but they don’t realise that almost every component of nextcloud can be uninstalled if you don’t want it. It can be pretty barebones if you want it to be.

I don’t see how you’re going to get away from the domain requirement, though, unless you’re just going to connect to your raw IP address? (And if you are, I didn’t think that nextcloud required a domain?)

Sorry I can’t be more helpful with a different solution, but hopefully someone out there knows of something that would suit your needs.