Here is a possible solution, although it assumes the parenthesis will be in the first list element. I've attached my test workbench that you could copy the transformers from.

I would usually do something like regex to replace the delimiters outside the brackets with a different delimiter with a StringReplacer and then use that in the attribute splitter

I think this should match semi colons outside of brackets

;(?=(((?!\)).)*\()|k^\(\)]*$)

This worked! Thank you very much!

@bgossett127​ or a single StringReplacer with the regex ( \\(EXT:(T 0-9]+;?)+\\)) replaced with <blank> to remove the extensions and then followed by the AttributeSplitter to split the remaining string