I've been using Betterbird for a good 6 months and it's the best email client I've used, it's what Thunderbird should be out of the box. With the Proton bridge app running in the background it integrates very well.
It's Thunderbird but with bugfixes and additional features. Bugfixes I like are being able to sort by attachment and minimize to my tray. Features I like are regex searching/filtering (including encrypted messages), opening to the same folder every time, being able to change message headers, being able to directly open links in messages I'm writing, maybe a few more I'm forgetting. Regex searching is the top used additional feature for me.
Since BB isn't a hard fork of TB, it stays up to date with bugfixes and features that new TB versions include, and they often restore existing features that new TB releases break or remove (at least 4 in the last major release v115), and are open about breaking features in new versions (like IMAP folder corruption in both TB and BB v128.0 that they say they hope will be fixed in v128.3.0).
Try Claws Mail too while you're at it, if you're on Linux. Most powerful email client on Linux, with everything OP mentioned (some of it available as plugins, but your distro should install those too).
I saw the popularity on AUR several times and installed but couldn't figure out why people use it.
Their website cites GDPR or whatever and is honestly kindof meh.
I was interested in the conversation layout, advanced email headers and recoloring things like folders but idk if they have that or not.
Also I've always been annoyed that half the dialogs are integrated tabs and the compose email is a window that can't be nested. I can't remember if they fixed that or not.
I wasn't personally impressed by the logo, it reminded me of a 2000s Mozilla but maybe that was intended. It didn't have enough distinguishment from other Mozilla products while communicating that it was for Email or related to Thunderbird. It honestly looks like a browser.
I've been bitten by some popular but abandoned AUR releases. Lately I've using yay -S instead of pacman -S with yay config defaulting to AUR and haven't had too many issues. Just one where my modified OBS uses an ffmpeg branch that conflicts with the VLC one and I haven't dug in to the package management weeds deep enough yet to learn if or how I can have conflicting packages installed but with isolated scopes.