I was looking for a used audio device with at least 8 line/mic ins/outs and phantom power supply, that would run flawlessly under Ubuntu 20.04 for recording (e.g., with Ardour), SIP phone conferences, and basically everyday stuff when I want headphones. I want my basic instruments and microphones plugged in to be ready to use when I fleel like it, and just have to switch on the device to be ready to go.
Spoiler: I haven't found that yet. But with the rack version of the M-Audio Fast Track Ultra I've come quite close.
It took me about two weeks of searching the web and trying stuff out, until I found a setup that so far I think I can work with. So I decided to share my solution here, may it be helpful to others in a similar position.
I have tested this configuration with both the 19" rack version ("8R", udev idProduct 2081) as well as the standalone device (udev idProduct 2080).