I don't think TB looks dated. It looks functional. Which is exactly what I want from a mail client. If I want something that looks modern but is dysfunctional, I'll just switch to Outlook.
* 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.
I had the 2-line message header view on my wishlist for a long time. Thunderbird did pretty great.
As I posted above, there is a setting for saving sent emails in the inbox along with received messages, so that threads are complete.
This.
For better or worse, I've rather come to view "modern" flashy UI aesthetics as being indicative of poorly performing software. It's not fair, I recognize, but in my personal experience, it's a reasonable heuristic.