Seamonkey or claws mifht be good alternatives too.

http://seamonkey-project.org/

https://claws-mail.org/

How is KMail and Evolution at this point? I have not tried them in like 10 years. Are they actively maintained and a real alternative for serious email use?

Both are ok last time I tried (last year?) but Geary is default on Gnome distro's now I think [0]. Geary is much more minimal though.

I myself am pretty spoiled by Protonmail I think, really enjoying that.

[0] https://github.com/GNOME/geary