Went ahead and updated to 5.0. And immediately hit a "No update required" bug, had to shell in and disable the cache. Great testing, Automattttttic.
Took a look at Twenty Nineteen theme, but even after completely customizing the CSS, it was screwing with my layout in unpleasant ways. In 2019, blog columns are 640px wide, instead of 840px in Twenty Sixteen! Nope. So updated 2016, restored my Fake Emoji Fuck Off, more custom CSS, and things are back to normal.
I'm ignoring Gutenberg for now, but Jetpack's got a Markdown block, so that may usable someday.
Am I really better off with this giant pile of junk, instead of going back to blosxom or some other static generator? I could write my own in a week. Well, I would miss media management. Automatic reposting to Mastodon and Twitters; I could script that myself, too, but Twitter's hostility to APIs means I'd be constantly fixing it or cutting off Twitter. So I'll stick with this for now.