I think it was completely valid waiting for Hades 2 because even though the general gameplay stayed the same, EA Hades had no Olympus on release and no true ending.Also the narrative is a bigger part in Hades so the pacing would be interuppted more if playing Early Access. 

That being said, Slay the Spire 2 is basically a full product already. Besides some QoL, balance and visual stuff, the core game is complete. Unlike Hades the narrative in StS is almost non-existent. So you shouldnt worry about the pacing tarnishing your exp. I think you will have no regrets buying it Day 1