Ugh, this ended up taking an absurd amount of my time. The fruits of that dubious labour are available now, though: Swift code syntax highlighting in WordPress.
TL;DR: they pretty much all do a poor job, although Code Block Pro has the most promise, and SyntaxHighlighter Evolved gets an honourable mention as a relatively lightweight "dumb" plug-in that does a surprisingly passable job nonetheless.
If folks in our community, like @jtbandes, continue doing the hard work to improve Swift's syntax-highlighting support in Visual Studio Code (and on GitHub), then those benefits should eventually make it into Code Block Pro as well, as they all share the same language grammar definitions. So there's perhaps room for optimism that the situation will get better.