Ereader

Good question. I used my kindle in the same way, and I found Koreader was just more feature rich (and snappier). The dark mode I was missing was back and when I opened it, it went straight to my library instead of a dumb storefront. It might not be worth the effort if you are satisfied with your experience of a kindle in airplane mode with your calibre library on there, but I enjoy the features and the open source aspect.

(Edit: I forgot to mention that I now use a Kobo Clara color after my old oasis died)