Painting work in new construction by own effort

I would only fill in. If cracks appear after some time, fill them in again, paint. Fine cracks are covered this way, larger ones also show in the [Malervlies].

In 9 out of 10 cases, it usually doesn't work the first time.

It's certainly not rocket science! With the right determination, you can definitely manage to be part of the 1/10 ;)
The prerequisite is that you want to do it and don't give up right away at the first strip.
Watch 2-3 videos on YT, you'll already get some basic tips there. Buy decent paste + wallpaper smoother/wallpaper spatula and get started :)

Malervlies from Erfurt is probably one of the better-known brands, I used the 150 g which is relatively stiff. It could have been less. Otherwise, check out, for example, AS Meistervliestapete on eBay, with which I also had very good experiences and results...

Depending on the Q2 result, you don't need to putty anything else; just go over everything with a sander to smooth the surface and get rid of pimples, etc.
Apply primer, then later paste on the wall and put the Malervlies on. If all else fails and you don't succeed or like it, peel it off again without residue and consider alternatives. After all, you have nothing to lose?

then paint the wall

painted walls

You mean painted walls?! The artist paints, the painter or you yourself paint ;)

Applying painter's fleece all over the Q2 plaster doesn't make anyone happy either because the conditions for it simply aren't right.

Yes. Q2 is precisely meant for applying something that masks the later cracks and the current small unevenness in the plaster.

I assume that I will have to putty over it again and sand it down to possibly remove a few pimples or small holes

So: we also painted the Q2 ourselves. We refrained from painter’s fleece because we are not wallpaperers. However, I have been painting all my DIY artisan life. We were lucky though: I can’t imagine those small pimples mentioned; we don’t have anything like that. Or nothing that could disturb the appearance. We have lived in the house for 8 years now. The cracks appear, although only visible from very close. And in corners of beams. One beam is continuously cracked; we have to/want to do some repairs there. Personally, I think little of "just sanding over it" in EL, because then you affect the plaster texture. That means: you would have to sand the entire surface, not just a small spot, because otherwise you will see it on the surface. Everywhere you additionally work on, you will later see a finer texture on the painted surface. You have to ask yourself what you want: a smooth and painted wall or a plastered painted wall. Those are differences. We wanted the industrial style and so you can live pretty well with a small hole or wave on 8sqm of wall. The places where we will soon repair with wallpaper are the drywall on the slopes and the masonry gable side, because you can see quite a bit there in the grazing light. I hope that today’s wallpapers are easier to apply than those from 20 years ago.

Personally, I don’t believe in "just sanding over it" in EL, because then you affect the plaster texture. This means: you would have to sand the entire surface, not just a small spot, since you would see it on the surface. Everywhere you additionally intervene, you will later see a finer texture on the painted surface.

If you want to wallpaper over it, you should indeed "just sand over it." First, otherwise you see the small bumps, and second, you definitely won’t see the sanded spots behind the wallpaper ;)

We also received Q2 from our general contractor.

Afterwards, the house was puttied to Q3 by an acquaintance and then painted immediately. The acquaintance does this professionally as well.

We think it looks nice; I don't like anything that has to do with wallpapering.

We have fine cracks here and there, but we prefer to putty, sand, and paint rather than have fleece or wallpaper.

We have about 153 sqm of area, and the acquaintance completed everything alone in about 14 days.

For us, it is no problem to sand the wall beforehand and fill holes so that it is as smooth as possible

Optimal would be to create a smooth wall and paint it ("not paint :D")
if we can't achieve a smooth wall or it's not recommended, our first alternative would be roll plaster (easy to apply and hides a lot) or brush plaster

the last alternative would then be painter's fleece (since we have not had good experiences with wallpapering so far)

The most important thing again is that if I do not use painter's fleece, the wall after e.g. 1 year should not look like Swiss cheese :D
and should not have cracks all over the wall

after 1-3 years, filling individual larger cracks and repainting is normal and okay but the whole wall should not be covered with small and large cracks