I started switching over to plaintext files with Markdown a few years ago. It’s been great for interoperability. I don’t need to use a heavy program on my desktop or phone. I can view, edit, whatever I need as long as I have terminal access.

I switched with Bookworm. It’s great!

Something worth noting is that F-Droid is both an app to download other apps but they also maintains a repository of apps. You can use alternative store apps (like Droid-ify) with the F-Droid repository OR you could use the F-Droid app with a different repository (like IzzyOnDroid). You can mix and match to meet your needs.

I use the Droid-ify app with the F-Droid, IzzyOnDroid, microG, NewPipe, and Collabora repositories.

Once you start down this rabbit hole, give Obtanium a look.