As in title, preferably open-source, but doesn’t have to, I will start:
KOReader - Amazing reader app, and being able to use the same app across various devices, even non-android ones, is pure bliss. Japanese support is pretty good too, which makes me happy. Though… Sadly no vertical text support :c
Syncthing-fork - Being able to keep a library of books, and some files, and easily keep the shared folder across various devices, is mega comfy. Tachiyomi - Cute manga reader.
NewPipe - Really pleasant youtube client, that i sometimes use to listen to things in the background.
Kaku - An useful Japanese OCR app, that works… quite decently for when I encounter kanji that I do not recognize, and is not text.
EinkBro - A web browser, that on e-ink devices, is quite comfy. Has few… quirks, but works well on e-ink
I find that Neo covers most of my needs. Although there is one thing in particular that I miss: swiping actions on seperate icons to launch another app/action. I haven’t used Nova in a while, but it had nicer previews inside the options, and I find that Neo has a few minor bugs that annoy me at times. But Neo has the things that are most important to me: multiple pages, tabs or folders in the app drawer, icon pack support, search and gestures (not on each icon, as noted earlier). I also miss the ability to place icons between the defined grid as in Nova.