This is really cool, and actually I think LLMs would be a great fit: they can generate your constrained language from more freeform language input by the user.

This is really cool, and actually I think LLMs would be a great fit: they can generate your constrained language from more freeform language input by the user. Might make it easier to get into, and you've done the hard work already by specifying a language for the card rules