The real issue still stands, I can't get the "thumbnail browser" or the "left main panel" thing to sort my music by works unless they are m4a files as shown in the last image I posted. Or maybe I can but I have to use virtual tags? Maybe I can combine the two work tags to one virtual tag?
I don't use m4a files myself, but doing a quick test, I think Picard writes @wrk for work, and MusicBee recognizes that just fine for me.
If Picard doesn't write @wrk for you, that's probably something to investigate.
(e.g. maybe 'Use release relationships' and 'Use track relationships' are not checked?)
In MusicBee, indeed Work doesn't seem available by default for the thumbnail browser.
So you could either make a wish for that, or as you suggested yourself, create a virtual tag for it.
This should work:
Label it something else than "Work", e.g. something like "Work (v)", and use this as formula: $IsNull(<Work>,,<Work>)