Rusty heating water with discolorations discovered.

I will probably never get that out again, or does it go away with flushing and refilling?

With each flush, you introduce oxygen into the system, which again promotes rusting and the growth of bacteria.

There are companies that specialize in cleaning, you should get informed there.

So a normal rinse with refilling isn't enough then, right?

No, as I said, that could possibly make the situation even worse.

so just install magnetic and microbubble separators and wait to see what comes out of the heating water analysis. We do not have the result yet

Yes, and enter this into the internet search:

bosy Korrosion und Schlamm

There is quite a bit to read about this topic.