The pdf-reader used by disroot is fine, but it does not allow viewing of epub / cbr/ cbz files, while this one does https://apps.nextcloud.com/apps/files_reader
Could that be activated/implemented?
Although it does nnot remember where I was last in a book, it does allow me to keep my notes centrally and it does allow reading on various platforms in the browser. It also allows having a quick look at books and comics.
Its git-page is https://github.com/Yetangitu/owncloud-apps/tree/master/files_reader
I have a free account at ocloud.de and they have this viewer as well as onlyoffcie which works well. Onlyoffice is a separate thread,