High-quality painting tool, which brand?

According to the product description, yes. If necessary, consult Baustoffheini again.

ah ok thanks. Would that be the Knauf Betokontakt?

According to the product description, yes. If necessary, get advice again from the Baustoffheini.

ok I will ask about it again. However, any reviews I found are based on Tiefengrund and not Haftgrund :D I am confused :/ I have not found a single case where Haftgrund was used for the concrete slabs.

Primer is intended to consolidate/homogenize strongly absorbent or sandy substrates. So that you have an even (and slightly reduced, so you don't need to apply layer after layer) absorbency for the next coat. You need this, for example, on an already plastered wall. Otherwise, the plaster will absorb the paint. Bonding primer is meant to establish the adhesion so that the plaster sticks well, especially on substrates with low absorbency. Concrete is such a substrate. Theoretically, you can then apply the primer on the putty at the end to achieve a more even appearance. But it is probably unnecessary because the putty is applied only very thinly.

Good morning, I have successfully completed the first test wall/ceiling and am very satisfied with the result. The only thing is that the coverage of the Haering Deko Quartz is not very strong. Therefore, I will do a first coat after the filling. Do you know a good and relatively inexpensive paint that would be suitable here? The Dekoquarz is free of pollutants and the paint should be as well. Can Alpina be used for this?

The opacity of the Haering decorative quartz is just not that strong.

Single or double rolled? Colored or the not so nice "white" of the plaster?

I basically threw it from the bucket onto the wall. I think the color is okay. I guess I just rolled it :D