Hi, after using a few unsatisfactory solutions, I discovered Trojitá, which meets my preference for being just email (no calendar, tasks, etc.), and is quick and light. I does have address book and spellcheck features. It is IMAP-only and the communications with the server were optimized as part of the developer's thesis. It was written for Qt5 and works well on my on a Pinebook Pro running Gnome-Wayland.
One thing I like is a more elegant way to block or allow remote images. It does block them by default and puts a notice at the top of emails. Unlike Thunderbird, this notice is small and scrolls out of view as you scroll down, instead of continuing to take up screen space. Geary is the only other program I've used that doesn't waste much of the screen, but on my Pinebook under Wayland, Geary would crash the computer to a black screen when I scrolled using the touchpad.
I'm using the version directly from git. For me, the official package always had spinning activity indicators and upon opening an email, it would display a large "fetching message" image over top of the message, and that image would not go away. The trojita-git AUR package appears to be long out of date; there's a comment from 1.5 years ago that says it is pointed to an old mirror, and indeed when I tried to install that way, the download failed.
EDIT to add new things I've learned:
- It appears to use only about 40 MB of RAM!
- The method cscs posts allows a person to use the package manager and get the program fully installed while getting the same version as git. Git works but it's more manual and the program is run directly from its folder instead of being in the system menus.
- The address book is weak. It's abook which seems not to play nice with CardDAV and other interactions.