@Christoph welcome!
Heat sinks help for sure. In Taiwan, I was told Maxway presses a slug into the headtube to weld, then press it out. No reaming needed. The slug absorbs some heat and also shapes the tube. I wonder if we are doing it all wrong (starting with undersized bores and reaming them). Sure, pressing in and out a slug takes time, but so does reaming.
The other angle you can look at is that your headtube might be too thin for your welding method and technique. I find that Italian tubing manufacturers often push the wall thicknesses too thin. A machined headtube will have much thicker reinforcement at the bearing seats, making it less prone to warping.