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.