There is a brand new plugin for KOReader that offers a richer experience and uses Kavita's API. I don't have a link handy, but it was shared in the Kavita discord (and will be on the wiki once I write a new page for it).

Koreader's OPDS implementation is VERY rough around the edges. It doesn't support much of the metadata and doesn't follow the spec very well. I had to write hacks in Kavita to give users better support for it. (My understanding is Koreader isn't too hot on OPDS in general).