%genre%/$if(%albumartist%,%albumartist%,%artist%)/%album%/$if(%discnumber%,%discnumber%-)%tracknumber% - %artist% - %title%
In other words:
→ Album artist if available, else, track artist
->->-> Disc number if available, followed by track number, artist, and title
Play around and see which of these tags you really need, and for the $if functions, see if the relevant tags are always going to be present (which would be up to you), in which case you can do away with the function and pick which tag you like.
The $if function returns the second value if the first one is not empty. Otherwise, it returns the third value, or if not given it returns nothing. I reckon you thought you needed it. You don’t, in fact here’s a simplified one for you:
This one would do what you expected. But you won’t get the automatic niceties of my version. Play around and see what you need.
Also, report back, I didn’t try any of these out because I’m in bed