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:
13da2c2f-cb18-48c9-b90c-950c44bf2770-grafik.png

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.