Your comments

Ubooquity not having a mark as read/unread feature with some little "read" icon on the thumbnail of read books/comics is the main reason I've tried every alternative to Ubooquity but wasn't satisfied with any of them so I just stuck with Ubooquity anyway and just used some external solution to try to keep track of what I've read. I definitely wouldn't mind paying for a paid version of Ubooquity if it had that feature. Or giving it a go myself if it ever becomes open-source.