Librera Reader is the best reader I have had the honour to try. I have been using Librera Reader for several years now, and I think it is safe to say I love the app. It is actively maintained, and new features are added continuously. There are plenty of settings to allow you to modify the user experience exactly to your liking. If you are considering reading any e-books or PDFs on your Android devices, give Librera Reader a try. It is a fantastic app.
K-9 is rebranding as Thunderbird for Android, indeed. But since many users of K-9 expressed their desire to keep the logo and name of K-9, Thunderbird agreed to publish both versions: one rebranded as Thunderbird for Android, and the other keeping its original branding as K-9. We will see whether that will complicate things for Thunderbird team too much yet.
Beside concrete suggestions, I would suggest having a look at the applications you use. (Decide whether you actually use and need them first.) Search for the application on AlternativeTo or similar sites and look for a FLOSS alternative there. And also search the internet for general FLOSS replacements for the use-cases of the applications you use. No need to rush anything. Even replacing one application a week is good progress.