Applying smooth plaster on drywall panels

I had also said until a few weeks ago that Q1 is enough for brush plaster. But then I saw a ceiling that was somewhat better than Q1 but also not really Q2, with 1mm grain brush plaster applied. You could see every void and every unevenly drawn edge. A good Q2 would definitely have been necessary there.

Thank you very much for your answers!

I have now googled Q1 and Q2! Do I understand correctly that in Q2 the bumps and screw holes are filled and sanded and the rest of the drywall sheet is untreated? If that is the case, I would feel confident doing it and could save costs again that way!

Yes, that's how it is.

That's exactly right. As already said, once putty, sand, and apply your plaster base.