Excited to hear about Hardcover! I like StoryGraph but the lack of API frustrates me - I want to be able to sync back to my general notes store (Obsidian). Hopefully Hardcover works better with that.
GR ditching their API was one of the primary motivations for Adam Fortuna (Hardcover's founder and Lead Dev) to even think about trying to create a competitor, so when he did create one, having an API available to others was a primary focus.
Note: Hardcover is also working towards open sourcing at some level, hopefully in 2025.