I have a Kobo and use calibre to manage my books. I don't sync online it's just plug and remove what I have read and upload my next lot of books that I want to read. My Kobo currently has around 300+ books on it at the moment so I can read whatever I feel like it. Doesn't help your syncing between different devices but I guess you could us Dropbox or something like that to link up your phone, tablet etc.

Yesterday I installed Grapheneos so I've swapped to florisboard. Not bad but it's very specific on where you swipe using gestures. I'm slowly getting used to it but gboard was a lot more forgiving.