Even if we do not official support an Debian upgrade, following link might be helpful https://dietpi.com/forum/t/qbittorrent-version-shipping-with-dietpi/2326/9
btw, are you sure you are not able to update NextCloud via Web GUI? I did a test installation on Stretch and it was going to install v21 without issues. At least the installation seems to ignore MariaDB 10.1 