Recommendation for the construction of attic insulation

Thank you very much for your detailed and very helpful answer once again!

I am only confused now about the question of which specific construction is possible or recommended in my case (i.e. without a concrete ceiling and with an existing vapor barrier). What I believe I have understood so far:

Construction option 1) Remove the floorboards, take out the insufficient glass wool insulation, install a vapor barrier and seal it properly, etc., use mineral wool cavity insulation to the full height of the joists, rough boarding, walkable mineral wool boards, chipboard panels

Construction option 2) Leave the existing construction as is, walkable mineral wool boards, chipboard panels (but not sure, since you also said that you would be cautious with mineral wool boards without a vapor barrier and chipboard panels are not diffusion-open there either?)

The first option seems to me to be the clean/"proper" one but more complex. How can I insulate my intermediate ceiling without having to dismantle the existing construction and the existing slight (but at least) insulation? Since there is no vapor barrier, presumably only a diffusion-open solution is possible?!

For the roof, we currently only have Heraklit under the tiles, so I could simply do a "classic" cavity insulation with mineral wool there and then a vapor barrier foil inside, right?

The stairwell opening is in the unheated stairwell, i.e. I will definitely seal the existing hatch there, but I don't think I need to make any special effort.

What I believe I have understood so far:

you have understood correctly. However, building on the existing floorboards only makes sense if the rafters are completely filled. Otherwise, you have an air gap in between that will definitely be ventilated somewhere - everything lying on top of it will no longer be insulating.

The first option seems to me to be the clean/"neat" one, but it is more complex.

Yes, that is usually the case :) botched work is always easier.

How can I insulate my floor ceiling without having to dismantle the existing structure and the existing slight (but at least some) insulation?

If, as described, the cavity between the rafters is fully filled, you can simply use wood fiber with a compressed surface on top without additional chipboards. Or non-walkable mineral wool or soft wood fiber mats, but these are then not walkable at all – in return, 100mm thickness costs maybe 5€/m².

If the cavity is already filled (I estimate the rafters to be 140-200mm thick), I would add another 100mm walkable wood fiber on top, as it is the simplest – honestly, though, 140mm fully insulated is not bad at all. Additional insulation will not change much.

With the roof, we currently only have Heraklit under the tiles, meaning I could simply do a "classic" insulation between the rafters with mineral wool and then vapor barrier foil inside, right?

Yes, exactly. However, the area to be insulated is significantly larger than the floor, and you won't get the gable sides warm like that. I would rather fully insulate only the floor sections – unless you want a warm attic.

Are there slopes on the upper floor? Are they insulated? Can you get to them from the attic?

OK, slowly a coherent picture is forming for me ;) That means, to be absolutely sure: If the cavity is filled, I can simply use wood fiber with a compacted surface. And then I don’t need a vapor barrier at all (it is recommended almost everywhere, hence my surprise)?!

Slopes in the upper floor: Yes, unfortunately they exist and unfortunately you can only access them from the outside. I’m not sure if they are insulated. If they are, probably only with Heraklit boards...

Below the slope, the upper floor also has a ventilated curtain facade with little or no insulation (I couldn’t look underneath yet, climbing up was too risky for me). I will probably have to address this again (here the same question about the construction has also arisen for me), but apparently such a facade is generally quite advantageous from an insulation perspective as it is durable and has good insulating properties (once you have the 20cm insulation on, currently we probably have rather only a few centimeters).

If the cavity is filled, can I simply use wood fiber with a compressed surface? And then I don't need a vapor barrier at all (it's recommended almost everywhere, hence my surprise)?!

The outwardly diffusion-open construction does not mean that a vapor barrier foil on the inside is unnecessary.

There are certainly diffusion-open constructed roofs and ceiling floors; however, you should know what you are doing here and understand the physical background, temperature and dew point lines, and not have two left hands craftsman-wise and work carefully. Then it is a great solution.
It is important that the dew point is not undershot anywhere. But this can easily happen with incomplete filling of cavities and corners, and then you have a mess. With a vapor barrier, it's definitely safer. If you remove the old wool or whatever is inside now, do yourself a favor and conscientiously install a vapor barrier foil with an sd-value of about 3-10m.
If you simply place wood wool boards on top and nothing has happened so far, then nothing will happen afterwards either.