Why are files unusable outside of Nextcloud? Consider using the External Storage plugin.

Imo there are two types of file servers: smart clouds with offline and smart selective on-demand sync on brand-specific clients, groupware support, conflict resolution, and enterprisy plugins (Nextcloud, Opencloud, Seafile, etc); and dumb clouds with protocol-based file transfers and filesystem-tree/userperms instant compliance (copyparty, sftpgo, etc)

Of the first one, only Opencloud has a native-looking filesystem (PosixFS) and does it without dependency on a db. It supports smart sync for Windows (via the same API OneDrive uses). Linux smart sync is sadly nonexistent due to lack of protocols, and whatever other software do (e.g. using an .owncloud placeholder file) is highly experimental.

Of the second type, you’d get all the standardizations and speed but no bidirectional sync nor offlineness - again this is honestly an advanced undertaking requiring academic understanding of distributed systems and whatnot. On Linux you may try emulating some aspects of it via a half-smart client like rclone with VFS, but the UX to store files offline is still not there.

Knowing these constraints I’d tier my storage into 2 parts: the daily files like notes and recent photos stored in one of the smart sync solution, ready for download and later offline use; and anything unnecessary (Jellyfin media, archives, ) to be in a dumb SMB share/SFTP mount.