Starting with thermal imaging eye-candy.
That's a relative test, but it seems changing the fan might do the trick. I need to figure out proper mounting to conduct a prolonged test, but so far the pico seems to feel better with some fresh air. To check it, I was testing which part of the inside was the hottest. Previously it was the pico, now it's the heatsink of the stressed component.
Temperature before: 84° on the pico and 68°
on the GPU (no case mesh lid).
After (100mm CPU fan, thanks @BaK
for the idea); 15m of Furmark, pico seems to be always cooler than the GPU heatsink/backplate:
Actual temperature a bit higher than pictured because of the mesh, but the pico is not the hottest component (less red in infra) and the test doesn't crash after 13 minutes.
Furmark screenshot (click to zoom):