Thank you for the detailed feedback!
I'm sorry the upgrade system felt broken or confusing... to unlock those panels , you actually need combination of both panties and a specific number of girls and I'll work on making these requirements much clearer in the UI so it's easier to understand at first glance.
You are also absolutely right about the credits, so I have updated the credits to include individual arts to ensure they get the recognition they deserve.
I'm still learning and trying my best to be a good developer, so I truly appreciate you pointing that out, and I hope you'll give the game another shot!