LLM's are not predictable. The ZMachine plus any interpreter is. Also there are tons of libre licensed IF games out there, such as All Things Devour, Spiritwrak and a bunch more.

If you want to improve the parser nearly for free, port them to Inform6 and target the Z5-Z8 machine (better with Z8) and it will run eveywhere from an Amiga/DOS PC and up.

Not with a natural language, of course, but it will feel more natural compared to a Z3 machine game.