Hah. Well, after this thread, I was fiddling with Airmail’s settings and… I actually managed to take some of the cludginess out.
It had a lot of bells and whistles—especially in Appearance—that made the UX of the app clunky. After I slimmed it down (see the settings for Appearance I’m now using, below), the app seems to work a little faster.
It’s much cleaner-looking to boot:
So some of my qualms with it have been alleviated.
It’s still glitchy at times, sadly. (At least, I don’t think Send Later works at all, if the eight messages I just found in that box that were supposed to have been sent a week ago are any indication…)
But as long as I’m willing to tolerate a bit of buggery in order to get customizable actions, I think it’s the only app for me.