I’ve been hoping to find a non-PHP alternative to Nextcloud for a while, but unfortunately I’ve yet to find one which supports my base requirements for the file storage.
Due to some quirks with my setup, my backing storage consists of a mix of local folders, S3 buckets, SMB/SFTP mounts (with user credential login), and even an external WebDav server.
Nextcloud does manage such a thing phenomenally, while all the alternatives I’ve tested (including a Radicale backed by rclone mounts) tend to fall completely to pieces as soon as more than one storage backend ends up getting involved, especially when some of said backends need to be accessed with user-specific credentials.