It's a tactical nuclear solution, but - if you use any of those specific three, you should definitely check out KOReader, an open-source alternative firmware. I had a really easy time setting it up on my Kobo (and it's explicitly built to help people escape the Amazon ecosystem / recover old Kindles to utility); and there's a fantastic high-quality Hardcover sync plugin I use:

hardcoverapp.koplugin

See-also, if you're looking for a slick iOS/Android/web/macOS/Windows reading app, Readest has recently added Hardcover integration, and so far it's working pretty well for me, too. Although Readest is self-hostable, it's also a paid service if you'd like to just use someone else's cloud. Again, really slick, I feel it's a good companion to Hardcover vibes. (And it, too, has deep KOReader integration. All in all, a fantastic non-Amazon ecostyem.)

(Personally, I believe a really top-shelf API is the correct level of implementation, for this, for Hardcover themselves. It's a development-resources sinkhole to try to maintain an ecosystem of first-party plugins/extensions. But that's just my 2c as an OSS dev, haha.)

OK enough idea, but Readwise, the grandmaster daddy of them all, has this fully solved. This would be a cul-de-sac for Hardcover.