I'm the author of that app, it was made for a much earlier version of Owncloud and it would take more work to get it up to date with the current state of the platform than it would to start anew. It also was not made to support many of OPDS' more 'advanced' features like sharding and search. I've started to look into making a new version, possibly integrated with my other Own/Nectcloud app 'Reader', look for more info on that front in a few weeks.

Yeah, Marvin’s great. Pretty much the best ebook reader out there.

Have you checked out BicBucStriim? It’s a PHP based OPDS server that uses a Calibre library as its database. I’ve set it up on a Synology drive as that was most convenient, but it’ll run pretty much anywhere.