It's kinda wild that you don't need to be a professional engineer to store PII. The GDPR and other frameworks for PII usually do have a minimum size (in # of users) before they apply, which would help hobbyists. The same could apply for the licensure requirement.
But also maybe hobbyists don't have any business storing PII at scale just like they have no business building public bridges or commercial aircraft.
The cards _could_ be copyrightable, would probably be essentially a coin flip if you took it to court.
No individual card text (limited to just the mechanics) is copyrightable but the setlist of cards might be. It would come down to how much creativity went into curating the list of cards that is released. It gets especially murky because new cards are always being released and old cards are being retired, so they obviously put a lot of creative energy into that process. You'd have to avoid pre-made decks as well.
Unless you have funding from an eccentric MTG-loving billionaire, I see why you'd comply with the cease-and-desist.