Thunderbird absolutely sucks now, it is borderline unusable (it would not connect to our internal email system, and I had no patience with its incomprehensible UI to figure out why).
New Outlook is just a web wrapper.
It feels like there are far fewer options in this space than existed 20 years ago.
Does anyone still use Seamonkey?
I step over the OAuth with email-oauth2-proxy.
There are X frontends to NMH like Sylpheed or claws. They make sense.
I've had a few issues with it for sure but still on it as nothing compares. Definitely not for the price. You didn't say what's incomprehensible about the UI, it seems fine to me. I don't need my email client to look slick(subjective anyway), I just need it to work and work well.