k9mail for Tablets/Chromebooks: Enhance Tablet Mode

Introduction

Love k9mail on my phone. Thought it would suck on my shiny new Pixebook because it's engineered for smaller displays, but it doesn't and I believe it is the best email app for my Pixelbook.
It does what it says on the box:

• Easy to setup and integrates well with contacts and file storage.
• No 3rd-party data collection.
• It is the only email app I have found that can delete emails with the Pixelbook's "backspace" key. The "refresh" and "back" keys also work (which is handy because some of the settings pages do not have a back button to touch)
• it also works in windowed "phone"/portrait mode although the window is quite small. Almost like a widget!
  

To make it even better Tablet/big-screen mode could be enhanced.

Expected behavior

Would be great to capitalise on extra screen real-estate/space on tablets and Chromebooks.

1. allow 3 columns? Folders/Accounts, messages and preview (multi-column view for settings would also be handy)
2. include icons for all tasks (notably: reply, reply all, forward)
3. a "back" button or "close" button (X) for settings.
4. WISHLIST: support split-screen (re-sizeable window). A larger portrait view would be handy for split-screen. eg. 3 columns in full-screen landscape mode, 2 columns in split-screen, 1 column in phone/widget mode.

To be honest, that would be enough!!

Actual behavior

1. lots of wasted space and tedious going back and forth from folders to messages. Less of a problem with the Unified inbox
   
2. have to click on the send icon and then select the action every time. Twice as many clicks as needed.
3. Once you have selected one of the menus have to rely on the "back" button (<-) or "esc" on the keyboard. In tablet mode you need to swipe up from the bottom and use the little "back" arrow that appears bottom left.
4. You can currently only use k9 in full-screen or mini-phone views.

Thanks

Thanks for your time. I appreciate all the effort you put into this app. I hope none of the above comes off as pure criticism, it is my intention for it to be wholly constructive. This is the first time I have been inspired to post a feature request on GitHub and it's because I think you have an excellent App.
I would even be prepared to pay for a tablet version of K9. It would make that much of an improvement to my Pixelbook.

Environment

K-9 Mail Version: 5.403
Android version: Android for Pixelbook

No files, tests, or entry points are named. Start by locating the current tablet and Chromebook layouts, message-list navigation, settings navigation, and reply actions. Compare the existing behavior with the four requested areas, then define a scoped tablet-mode change and verify the resulting column layouts, actions, navigation controls, and split-screen behavior.