A bit ranty, but I occasionally see topics relating to overwrites and conflicts. Might be useful to see:
Owncloud / Nextcloud - Tried twice a few years apart. Both times it was a disaster. After getting it setup and running, without fail, a few days later something would get deleted from a client that was never told to be deleted, a file “conflict” would happen that would affect/erase an entire folders worth of files, or something similar that affects data integrity. Also had issues with Windows desktop clients not syncing data up to the server. Syncing and data integrity is the core of cloud file sharing apps. This was and still is just a mess.