Maybe a little late, but here are some of the games I've found in my quest for good singleplayer card games. Note that I play on ios or steam mostly, so when I say mobile I know it's on ios but can't confirm android for sure. Also most of these games are a single purchase, no microtransactions aside from expansions maybe.

Arcanium (steam, mobile via netflix sub) - In this game you travel around a hex-based map fighting battles and collecting stuff. You have a party of three different characters that you choose from the start, each with their own deck that you can alter at any time from a pool of cards that you've collected during the run. I really enjoy the deckbuilding in this one, but I wish the runs were a bit longer.

Lost Portal (mobile) - This game reminds me a lot of the old handheld yugioh and pokemon card games. You build a collection throughout the game by buying, crafting, or looting cards as you move through a linear series of quests on a small world map. There a a bunch of different types of cards from which you can build various decks with different themes. The cards are most similar to mtg, but there are several differences, especially with how creature combat works.

Momentum (mobile) - This game is similar to slay the spire and dawncaster, except with each attack from yourself or the enemy you need to make accurately timed taps to ensure they go well for you. It's kind of like the quicktime event rpg combat in the mario rpgs on ds and gamecube, except they are absolutely required to succeed. Very hard game, but some may enjoy it a lot.

Thronebreaker (steam, mobile) - Someone mentioned gwent in another post, but not this. This is the singleplayer gwent game, complete with card collection and deckbuilding over the course of an interesting plot with hard choices. Exactly what you would expect from a witcher game and one of my favorites.

Legends of Runeterra (pc, mobile) - Technically not a singleplayer card game (although it is the least predatory and cheapest to get into by far). It does have a singleplayer mode called path of champions that is somewhat similar to slay the spire but with the runeterra card game for fights. Speaking of which, the gameplay is sort of a middle ground between hearthstone and mtg. I personally think it is better than both, having played all 3.

Dream Quest (steam, mobile) - This is one of the the og singleplayer cardgames. It was made by Peter Whalen, who went on to be one of the top game designers at blizzard for hearthstone and later at riot for TFT. It has what I can only call ms paint graphics, but the gameplay is solid and I enjoyed it a lot.

RogueJack (mobile) - A roguelike where you go through a dungeon and fight enemies by playing blackjack with a twist. Very simple, but pretty fun and different from a lot of these games.

Alina of the Arena (Steam) - Someone mentioned fights in tight spaces, which is a game I like a lot. This one is similar, although it's a hex-based arena. You fight multiple enemies at once by playing cards through the weapon/shield in either your left or right hand, altering the cards based on what you're using.

Ratropolis (Steam) - This is a weird fusion of deckbuilding roguelike and city-building tower defense. It's very similar to the kingdom games in theat it's a 2d city builder where you defend against waves of enemies, but everything is done with cards from a deck that you build over time.

Marvel Midnight Suns (Steam) - So the combat in this game is pretty good, card-based attacks with a team of recognizable marvel heroes and plenty of tactics during the fights. The production value is very high for a spinoff card tactics game too. The problem many people have with this game is it has this weird dragon age rpg map exploration and companion relationship system that massively slows everything down. Expect to spend more time reading dialogue than everything else combined. If you would be interested in a marvel themed visual novel then I guess that would be a bonus.

Some of the games I already see mentioned in this thread that I would also recommend: Monster Train, Wildfrost, Roguebook, Fights in Tight Spaces, Dawncaster, One Step from Eden, Across the Obelisk, Night of the Full Moon, Black Book, and Cultist Simulator

I've played all of them and they're all great choices, but this post is long enough so have a look at what others said about them.