okie I like this version a lot more (click the link to see it)

https://zone.dog/games/firmament7/

this version has the clouds draw twice fading in and out in opposite phases. the clouds that are in the invisible phase are re-sorted, thus the system is able to make gentle intermittent corrections to the draw order without being super obvious. I'm using stochastic transparency to do the blending because it turns out using alpha transparency makes the math much harder to get right.