Hey guys,
I’m one of the developers of DAVx5, an Android groupware app to sync contacts, calendars, tasks, notes and journals, and we provide access to WebDAV files as well. We don’t provide email however, and we don’t plan to 
Since a few years we also have our business version: “Managed DAVx5” - it can be configured via a remote config inside any Mobile Device Management software. Businesses and Organizations around the world more and more use MDMs to deploy pre-configured apps to their device fleet. Since we don’t have email support in our app we more often get the question from customers if there is a pre-configurable email-App on Android. Because this is a missing part in their (preferrably open sourced) ecosystem.
In our case we sell this MDM version of DAVx⁵ as a subscription service. And I’m VERY! sure that k9mail/thunderbird would have a many customers once this is spreading a bit!
I’m not sure if this was a topic for you, but I just wanted to bring this up. And thank you for your work!
Cheers,
Bernhard
It was a please to meet you and hopefully sharing our insights will help a bit in the future of your project!