Mozilla Thunderbird for Android is now available in beta, built upon the K-9 Mail app.
The beta includes core email features like account setup, email organization, and notifications, with feedback encouraged from users.
Thunderbird for Android will remain a separate app from K-9 Mail, requiring users to migrate if they previously used K-9.
I uninstalled it quickly after finding out it was trialware (missing features; popups to pay)
If I remember correctly, you should be able to just install the GitHub version.