Seafile is probably the next biggest/most developed alternative. When I’ve tried setting it up in the past I’ve had serious issues getting it running behind a reverse proxy. Would be fine for local or VPN/Tailscale style usage though. Also, data is stored in a proprietary database which would make configuring external backups a bit harder.
Syncthing is probably the most flexible option, but it’s not a “cloud” storage solution, it’s peer-to-peer file syncing. Doesn’t really allow for easy selective sync or quick deployment and configuration.
There’s also Owncloud but I don’t have experience with it.
I use both Seafile and Syncthing for various things.
I've managed to setup seafile ok behind traefik as a reverse proxy. Seafile has a basic web front end and allows you to integrate with Collabora for editing open document files. Its fine to just shut down the docker and copy the database I've found for backup. Or you can copy the files themselves, if you don't care about the change history (which I don't). I have as script to do this each night. Seafood isn't too hefty and runs well on an RPi, even the collabora integration does.
Syncthing is a really good piece of software and I use this to share files from my RPi to my phone, but bare in mind by default its send&receive both sides so its not suitable for backups in that state.
I use tailscale + tunneling, so rProxy shouldn't be an issue.
I already use Syncthing but looking something I can use to share links to file and access from public in general.
Seafile or FileRun seems to be good in my case.