I never played Slay the Spire or any games similar so I don't have a basis to compare
I played until the first level up, with Pomao and a character with a red hat thing that reminded me of red Cuban Berets, so I'll call him Revolutionary Bob
I didn't ever really use Revolutionary Bob since his skills weren't very useful. Pomao in contrast has the tongue slap move, which you can spam up to 3 times, does a lot of damage and costs no mana. It also transfers damage if you kill someone with it, which makes it 4 times better than anything else I tried from either Pomao or Bob.
I was able to easily clear Hard fights with this combination of moves, only taking a hit or two, and only using tongue slaps
I didn't know if there was a boss or anything - I thought that was the bar that was filling up, but it turned out to be experience. I leveled up and got more HP and stopped there.
I'd perhaps include some cooldown mechanics so the player can't just spam one skill, and instead it would focus on combinations of skills. I saw Revolutionary Bob had some poison moves, but I didn't know how they worked despite trying them out.
Perhaps add some skills where one is a setup, and another a payoff?
I also like the inclusion of using keys at certain points to buff your moves (doing 2 additional hits with Pomao when pressing E). It kept me active despite the game being turnbased, which is usually considered slowpaced.