I'm surprised no one mentioned Mutt + Maildir? I'm by no means a power-user but I really love it and find it very responsive for downloaded emails.
If you like Mutt, you might really love Aerc -- https://aerc-mail.org/
Aerc looks amazing, but I am still waiting for threading support before making the jump [1]. To the best of my knowledge, it supports everything else I would need.
[1]: https://todo.sr.ht/~sircmpwn/aerc2/94
My current stack is: Mutt, Neovim, fdm, msmtp, Syncthing, notmuch, lynx (for HTML conversion), mblaze [2] (for scripting), and a tiny pair of scripts to snooze and unsnooze e-mails. Here is an interesting observation, although a pipeline like this may look terrifying, it makes swapping in Aerc to take it for a spin trivial since it all interacts with a Maildir.