If you liked the all in one solution like Seamonkey, You can also try Vivaldi Browser which created by people who built the original Opera Browser.

https://vivaldi.com/download/

Have you tried claws mail.

https://www.claws-mail.org/

BTW: It’s GTK based, so it looks not very native on windows.

Thunderbird has been great for me. With internal email systems, connection issues were usually due to proprietary protocols doing things in weird and undocumented ways that was the problem.

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.

I use mbsync, to Maildir and then NMH Maildir to folder/file format.

I step over the OAuth with email-oauth2-proxy.

There are X frontends to NMH like Sylpheed or claws. They make sense.

Before giving up on Thunderbird I would suggest asking AI to help you debug the configuration.