Nice one! I had a look through and saw your uncertainty about Replace "|" "$verticalBar()": web sources use | to separate values in searches or tag labels as you’ve already been using. If any data from the website response like a title or description includes | intentionally, it'll be mistaken for a separated value and mess up your results.
There’s other characters that can be troublesome like newlines and quotation marks but you can be inventive replacing them back and forth while processing:
UseFormat "$replace(%_current%,\n,#NEWLINE#,\\\",'''',|,$verticalBar())"
SayFormat "$replace(%_current%,#NEWLINE#,$char(13)$char(10),'''',\")"
Point is though, you don't need it for covers so you can safely remove that line but if you do need to extract metadata that's long and descriptive even if it's unlikely it's a good idea to use $verticalBar() as Mp3Tag uses that function to put back a | in Adjust Tags.
Only other thing I could suggest is given there’s not any other data like a title being returned by the response, the cover preview gets a bit glitchy and shoots off to the right-hand side without any other columns. The response does return various sizes for each image so you could implement a solution similar to this.