--device=all does not work for the flatpak version of Dolphin to enable the 1st party GameCube USB adapter on my installation of Linux Mint 20.3.
Again, running as root is the only thing I can find that works, but I am by far not a Linux guru so I may still be missing something.
For farther discussion, I'd greatly appreciate it if you directed it towards this 11-month old forum thread instead: