I've also tried through some Live images (
https://cdimage.debian.org/mirror/cdimage/archive/8.11.0-live/amd64/iso-hybrid/, standard and GNOME), but still the same issues as mentioned in the initial post...
Finally I've texted VirtualBox but they claim they can't help because the version I'm using is out of date (but it's the most recent I can use).
If I can't solve this by tomorrow, I'll look at Debian Stretch (as that's still lighter than Bullseye) and see if I can at least manage to work with that...
Thanks for all the help!