>I don't even know what such an ultra-flexible, low-context (= near-zero ramp-up time even if you only do it an hour or two a week, and some weeks not at all) but still high-paid gig would look like, or how to go about finding it.

There isn't a lot of daylight between low-paid, mostly uninteresting gig work and a real side job that takes some marketing, sales, avoiding time-wasting people who want to low-ball you, chasing payment, and some expectation of ongoing commitment.

I do have something drop in my lap now and then from someone I know, e.g. ghostwriting something. And that's fine. It's good money for maybe a day's work overall. But you can have tight deadlines and if you just wait for the phone to ring, you're probably not going to get a lot of that sort of thing.