Just some hypothetical considerations about the original problem you are trying to solve:
Suppose you would use 'Original Artist' for this purpose.
Could you make your formula work integrating something like:
If 'Original Artist' = 'Artist' (or empty), then… , else…
If that is doable, then you could probably solve the second issue by creating a virtual tag for the column browser to replace 'Artist'.
Using something along the lines of:
<Artist>; <Original Artist>; $RxReplace(<Artists: Performer>,"\([a-zA-Z0-9_ ]+\)","")
would probably work for that.
The original idea was to create a group heading to show tracks by the selected artist that have been covered by others.
e.g.
(https://i.imgur.com/mTgZpyI.png)
It is possible to compare Artist and Original Artist to determine which tracks are covers, with a virtual tag like this...
$IsNull(<Original Artist>,F,$If(<Artist>=<Original Artist>,F,T))
...but not to distinguish between cover songs by other artists and cover songs that the selected artist themselves have performed.
e.g.
This is not the desired outcome:
(https://i.imgur.com/nYw8voj.png)