This is my attempt, definitely not pretty but seems to work:
If(Regex([Album], /#(.*),#/ , 0),RemoveLeft(RemoveLeft([Album], Length(Regex([Album], /#(.*),#/ , 1))), 2),)
Basically I'm checking if there's 'something' after the last comma, and in case remove it from the album tag (using the length), and then remove 2 more chars (", ")
The album title format (the last bit) is always ", <<catalog name>> <<catalog number>>"
Example:
", Op. 127"
", Op. 18 No. 2"
", BWV 82"
", K. 515"
Hope it's more clear now!