Readarr was orphaned, so most people switched to using LazyLibrarian. LL is kind of difficult to work with and you'll typically see a lot of failures in the logs for various things. Instead, you could use Shelfmark which fulfills the same general purpose but is more straightforward.

For browsing/reading, most people use Calibre-Web. You can write a simple bash script to do periodic imports via calibredb. Just point the output directory of Shelfmark to the input directory for that command.

EDIT: I'm not sure if this exists in Headscale, but Tailscale also has Services you can setup so that something like machine.some-domain.ts.net:8083 can be mapped as books.some-domain.ts.net. For Kobo Sync or OPDS devices you can just download to local, but if you are just using an Android device, you can stream the book to the device via browser which is very nice.