You can also just sync your files. Koreader, by default, stores metadata in the same folder as the epub file. If you sync the directory with something like Syncthing, then you don’t need a server.

Buying anything on Amazon hardly seems viable anymore. There’s so much counterfeit crap there, and a million low-effort rebrandings of the same stuff you can get on AliExpress for cheaper.

Shop local when you can, and at least shop not-Amazon for the rest.