NGL, I'm such a huge fan of the game called NSFW Solitaire that I just have to risk offending and mention that NSFW Solitaire on Steam exists (for free), it even has built-in support for custom cards and backgrounds and such, and it was so easy that I actually made my own card pack using Dall-e 3 for giggles in like an hour. I'm actually upset that the best Solitaire game collection I've ever played has NSFW in the name and I can't play it during the day on my actual Steam account, and would gladly pay a few bucks if they had a zero-effort, exact copy just with the game renamed and with no NSFW content.
That said, NSFW Solitaire really IS just "all Solitaire card game formats, but with lewd (mostly vanilla) pictures, a bunch of weird porn-themed games to pick from, and a dumb puzzle minigame that nobody really likes to unlock basic-ass AI sex pics" so it's probably possible to do a bit more with the idea and carve out a different niche. Having your game let you unlock more art in a gallery for a specific character is alright, but it's gotta be special enough to make people want to play it. Card games in general (outside of gambling, like Blackjack + Poker) are already a pretty hard sell these days, I think only us old-timers even know what Solitaire is, haha.