That would make Mint unstable. That is exactly what unstable means in Linux context. There are debian based rolling-release distros, including Debian Sid. This is one of the reasons people choose Arch, because it’s a rolling release you never have to worry about version.
There’s a good chance you might break stuff by upgrading major version like you fear, and that’s why it doesn’t happen automatically. That being said it should be safe, but good on you to prepare backups.




How does it work on Android? One of my main use cases for Nextcloud is to be able to access some of my pdfs on my phone, the app seems to be focused on uploading which is something that while I do sometimes from my phone is much less often.