Regarding StS, I think you've given it a fair shot if you've completed it with three characters already. If you still want to press on, then I found the fourth character really fun and engaging. I didn't try that many difficulty levels (think I made it to 4 with the Ironclad), but I really liked the daily challenge as well to mix it up and quickly feel powerful in my runs.
I always think of Slay the Spire as my gateway drug into other deckbuilding games such as Monster train and Griftlands, so if you've enjoyed the card aspect of it, maybe give one of those a shot? (Or stay clear of them if you hated that part)
Annapurna is the publisher of those games. Outer Wilds was developed by Mobius Digital, whereas What Remains of Edith Finch was developed by Giant Sparrow.