I thought there would be more characters

Game is still in early access, and I believe dataminers already found data for more characters. I find it hard to believe they wouldn't add more before launch.

the original characters are more the same than different, really.

This I disagree with. If looking only at their starting relics, then sure. When looking at the card pools, they differ a lot. You have vulnerable builds for Ironclad. Silent has a proper draw/discard build now, and shiv strategies are a lot different. Status decks moved to Defect, and you have dedicated lightning and frost decks now as well as a new orb. The relics you get access too are different, you have different enemies to build around (many with very punishing mechanics for decks that are too focused), and so on.

The core gameplay loop is the same, and many cards and relics were carried over, but there are enough changes that StS1 strategies don't work anymore for the most part.

If you were looking for a drastically different game, I'd recommend looking at mods instead. For example StS1's Downfall mod changes it into pretty much a new game, and it's really a well-done mod.

I haven't played multiplayer yet though, so for all I know it sucks, though I've heard that it's fun from some of my friends.