Also have my own custom app, similar to Readwise, that backups up Annotations and highlights from my android ebook app (App/Moon+ Reader), Calibre, goodreads, kindle, google play books, notion. Very elaborate and hacked together but it's worked smoothly for a while now
Made some incredibly good progress on an API for the Annotation Department. This will enable moon+ to send highlights directly to the backend. Only problem I haven't solved yet is sequencing. App/Moon+ Reader sends highlights immediately after creation which makes concat actions difficult. I assume Readwise does it by collecting highlights before processing them but I have no clue how to implement that