>>2450107It's okay.
Four simple steps: I just 1) removed JPG artifacts with a noise filter designed for JPG noise reduction on grayscale images, then 2) reduced color depth enough to quantize the remaining jpg-affected pixels to their original state, 3) saved the image as paletted PNG and 4) optimized the resulting PNG file.
It's often possible to reduce filesize of certain types of images simply by converting them from JPG to optimized PNG even with all the jpg artifacts in them, because people simply don't know when PNG compression is far better than JPG. And it's lossless, too! But it's very rare to stumble upon an image that could be "recovered" from JPG noise this well (or even have a problem like this in the first place). This was pretty much a perfect example of a complex image that should've been PNG in the first place.
But the fix is far from perfect. As a downside of the color depth reduction, the lineart became slightly more jaggy with less smooth gradients, as you can see from the comparison I posted. Thankfully the image resolution is very high, so that downside is barely noticeable when viewing the image at normal zoom levels, 100% or less. The color depth reduction is by no means optimal, I just went with my gut feeling for something that would compress well, remove all remaining traces of jpg artifacting, but also leave adequate amount of anti-aliasing on lineart.
