"split-string" works well for this situation but for a case where there are commas inside some of the strings it would chop in the middle of those strings. So for the latter case and assuming the commas are consistently formatted, a safer approach might be starting in front of the first s-expression and then traversing to the tail of each s-expression with "forward-sexp" and check to see if a comma is there as expected and then delete the comma there (except for the last item with no comma) .
Yeah that's the classic comma in CSV problem. The solution is to indeed use a proper grammar/parser. The "forward-sexp" understands that as per major-mode rule as you say. Another more modern and declarative solution would be to take advantage of the full parse tree at your disposal thanks to tree-sitter (available in Emacs 29.1). Then you can simply do:
(let ((list (treesit-thing-at-point "list" 'nested))
(query "(list (string (string_content) @item))"))
(cl-loop for (_ . item) in (treesit-query-capture list query)
collect (treesit-node-text item t)))
