Ok so quick update here: I was following the mount procedure from the first link and it's looking good! However, I started with a fresh installation instead of moving my files around, but seeing as it worked out, migrating an existing installation should be no problem either.
So what I did:
deployed a new Cloudron with Nextcloud installed on a fresh CPX21 on Hetzner Cloud (kept it at 40GB though, might scale down if performance allows it)
mounted a 5TB Hetzner Storagebox via sshfs into /mnt/cloud and added it as Cloudron volume (/media/cloud)
mounted that volume into the Nextcloud app (uncheck read-only)
change the datadirectory in config/config.php from 'datadirectory' => '/app/data', to 'datadirectory' => '/media/cloud',
cp -r /app/data/admin to the mounted volume and touch .ocdata in /media/cloud
That's pretty much it I reckon, if you log in now you have this extra storage available:
I have both, uploaded a few GB manually and the rest via sync client, no errors so far. A restart won't change the datadirectory, so this should continue working after reboots. Encryption is also enabled and working as intended, still waiting for something that doesn't work properly.
To know the backup scope, you can sum up all the apps listed there + email data + box data. Then you would have to multiple that by the number of backups which are stored (eg. once every day for 1 week or so) This would be the maximum of storage required using the tarball method. For rsync, it would be much less since it would do incremental backups.