Discoloration of heating water (only the supply line)

And in return, they sell me the annual flushing and refilling as additional effort during maintenance?

That will only damage your system even more, because new oxygen is always added in the process and it is the oxygen that actually reacts with the water.

So then only DI water if at all and if it runs optimally the stainless steel MAG

Update regarding expansion vessel I looked into it and could not find more than the specification that it has a 12 liter capacity.

Currently I have a 12 liter expansion vessel for a heat pump indoor unit which has an integrated 190 liter hot water storage tank. A 20 liter series return buffer and about 100 liters of water content in the underfloor heating.

Additionally the content of 4-5 meters of 28 Cu pipe for the connection to the heating circuit distributor.

Is 12 liters in the expansion vessel enough? It somehow seems little to me... If the heating engineer would replace it with a V2A expansion vessel, could it be enlarged a bit?

But the V2A expansion vessel probably does not change anything about the issue, theoretically one would have to replace the buffer tank if it is only made of steel, right?