Ok, well I thought that all worked, but …
In the sys-vpn-mullvad AppVM (based on a debian-11-mullvad TemplateVM), which runs the Mullvad GUI, I am getting good up & down internet speeds. But when I link my Untrusted AppVM to sys-vpn-mullvad, I’m getting good download speeds but almost nonexistent (0.21Mb/s) upload speeds. I removed any of the firewall rules in the settings manager (using this post) but it doesn’t help.
Hmm.