Not to hijack but for non object storage use cases I’ve been using Grimmory[1] and like it. Opds, kobo api, kindle email and web reader cover pretty much every use case.
Not necessarily re: resource usage but rather: it does a lot more than what I have any interest in using. I just use it to expose books to my 2 ereaders (via OPDS) and to sync reading progress (via koreader-sync).
Grimmory is a web application first which is what I want. Access for my friends and, more importantly to me, an always online library with progress sync across all my reading devices.
Calibre still has some good use cases though like drm stripping and has a better workflow for heavy metadata editing, but my book sources generally come drm free and have good metadata.