The thing about slay the spire spiritual successors is that slay the spire does not need a spiritual successor. It's cracks aren't showing, it hasn't aged badly, and, most importantly, it's still supported by the developers to some extent.
Failure is largely inevitable if you simply remake a game you liked playing. A good spiritual successor should be born from seeing the cracks in a game you love that appear when you spend enough time with it. A good spiritual successor is something that can only exist thanks to the original laying the ground work. It should be a response to the game it's made after rather than a simple "yes, and'.
I'd say that if you have a clear and strong idea of where Downwell missed the mark, you're good to go.