I obviously don't begrudge the fact Thunderbird currently works for you. But there are a few issues with the current UI:
* 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.