Good points!
- Don’t use virtualbox guest addition iso. Guest addition are installed by default. If these don’t work VirtualBox Integration is the only way to fix it.
- Virtualbox guest addition iso will make things worse. (VirtualBox Guest Additions)
- Don’t install new themes.
- Don’t mix repositories. Whonix 15 is based on buster. My mixing with older repositories certainly things can go wrong.
Please compare with your local installation. Might include the same output. If not, please quote here.