just gave it a test, the design additions are introduced gently. Defaults for listings are on the familiar view. Menu quick actions exist to go from compact to normal to more padding.
I had the 2-line message header view on my wishlist for a long time. Thunderbird did pretty great.
Thanks, if I'm honest I had skimmed over that setting before, not understanding its purpose. (It also looks like they're now introducing a unified inbox, which is what I wanted.)
* Dark mode didn't exist for a long time, and I'm glad it does, but messages are still displayed brightly.
* The list of emails always has a columns and a header; there's no simplified vertical view.
* There's a threading view, but IIRC it doesn't combine sent and received messages into the same thread view.