Stockfish is a good reference point, an objective measure of how far the LLM's advanced.

It's not. Maybe if you used old versions of stockfish that predate the neural net methods used by current versions, because otherwise you'd be comparing the hand-rolled (by an LLM) position evaluation functions against an NNUE and the results of that are a forgone conclusion; stockfish will stomp it every time.

Maybe that's the result you want for some sort of rhetorical reason, but it would nonetheless not be an informative test.