I mainly just need basic file management features. I don't plan to share files outside of my tailscale VPN. I do need to support multiple users though.
I'm not considering Nextcloud because that seems too big. I'm also not considering syncthing for this project because I don't want copies on multiple devices.
I'm currently just using ssh+nautilus and that's worked great for just me, but something similar to Google Drive would be easier to onboard my family.
Nextcloud - full featured cloud suite (calendar, contacts, etc)
owncloud infinite scale (OCIS) or the Euro fork Open Cloud - the POSIX driver has a flat file structure and still supports users and shared data; OCIS is designed for larger installations, but running on a smaller, single instance totally works too
Since you rejected NextCloud, check out the other two. I'm switching from NextCloud to OCIS right now, and I may end up using OpenCloud if development looks stable.
Have not tried opencloud yet, but one thing that I find interesting is that it doesn't need a DB. Plan to test. Currently have nextcloud and find the installation a bit of a pain.