First, the update to 2004 takes ~3 hours.
The result with my I7 / 32GB Surface Pro 7:
The fan is still going, probably just a little bit better than before. Interestingly, the Core Temp App shows a core temperature of 42-45°C, which is just like without charger connected. The clock speed is mostly in the area of 1.2 GHz but with regular spikes to 1.7-2.8 GHz for short moments.
So, the upgrade to 2004 doesn't seem to be the solution. Still clueless.
Any more ideas?