@Zecc Thanks! That's an out-of-the box answer I wouldn't be able to come up with myself, and they seem to support all the platforms I care about and then some. I'll definitely consider this.
@Jaloopa Well, maybe not, if there's such a service that doesn't base the security on a phone number, doesn't Facebook the private keys, and has a real™ PC client (and not just an Electron app -- that would be exchanging one set of UX problems for a new, different set of UX problems). I've been looking for services like that. Do you have experience with Matrix, or Element, or however they call themselves now? Wire's desktop client was a real turn-off for me last time I tried it.
In a way, being transport-agnostic means that we can jump ship from one service as it becomes too shitty to use to a different one that's still good enough. We used to exchange encrypted 7-Zip archives, sending one-time passwords via text messages, until most e-mail services blocked encrypted archives for "safety" reasons. For the most part, I don't blame them -- they made it easy to escape AV scanning -- but banning them all the way is also terrible for privacy.
@pcooper Thanks! I'll see if there's any downsides to exchanging encrypted text files. I foresee newline-related problems (I should be careful to always send them with CR-LF newlines) and encoding-related problems (will Windows notepad auto-detect UTF-8, or should I use the ANSI code page of the recipient?), but those are all solvable on my side.
I'm also a bit afraid of PGP in the browser, with it constantly being exposed to websites we visit and the telemetry added with the updates.