I suspect they're also testing local conditions like radiation and whatnot.
Nah, the biological experiment capsule on the lander is way too small for that (it's on the lander platform, not on the rover). They'll probably crossreference with LND though, the radiation sensor suite contributed by the German Space Agency on the lander.
The lander basically only carries LND, the biological experiment capsule and a radio frequency spectrum analyzer mostly to detect solar storm emissions (being shielded from interference from Earth where it is; there's a Dutch-Space-Agency-contributed radio astronomy instrument for similar reasons on Queqiao, the relay satellite).
Hydroponics underground would definitely work... not really sure what they're trying to prove honestly.
Plant growth is rather sensitive to gravity actually, especially plants that grow "bidirectional" (roots and stem).
There are also issues with testing certain automatisms in habitat environments, for hydroponics the water is an issue in particular; less the cycling, more the fact that you can't test that kinda thing anywhere where NASA gets involved because they want prohibitive triple encapsulation on any water-related experiment going to ISS (hence why e.g. Eu:Cropis tests this on a fully separate satellite, which has been waiting for SpaceX to launch it for two years now).