Just for the purpose of documentation, if someone finds this: As an alternative to Nextcloud, I have decided to use Radicale for Calendar and Contact synchronisation and Syncthing for files, where I sync these with versioning to an HDD volume attached to the server. Now I am still looking into how to implement an external backup of the volume once per day, probably I will use rclone or something similar with a cron job and sync to another provider.
So far, this works, and the GUI for volumes in Cloudron gives me a graphical interface for my files in the cloud anyway if I want to, just without the possibility to open them in an editor.