Construct attic floor

I have actually already considered leveling compound. I just didn’t deal with it because I don’t know if I would have sufficient impact sound insulation from it, and with the dry screed panels I would have chosen ones that have something underneath on the bottom in the hope that you then hear the footsteps less from above.

There is, for example, vinyl flooring with impact sound insulation directly underneath.

I don't quite understand the "waves" issue. The current boards are screwed down and strong enough to walk on, right? Maybe you can improve that by adding something underneath as needed, or how/where are these waves?
Similarly, you could screw down an additional layer of OSB boards as the floor and then level it out at the necessary spots and lay it without waves.
You will hear the "steps" underneath anyway; for me, impact sound insulation for noise reasons would be rather secondary for your project.
I would glue painter's fleece onto the OSB boards on the slope anyway and then paint white/light over it.

Only a structural engineer can ultimately answer that for you. You probably still have the exact construction plans, so I would consult a specialist with them (load-bearing capacity of the attic floor). (Dry) filling, (dry) screed, and covering already weigh quite a bit in total. So currently, there is only one layer of OSB board? We also have a wooden beam ceiling, then some slag in between, simple floorboards, OSB board, and then laminate directly follows. I wouldn’t have trusted another structure in our case without expert advice. It’s not just about how much the beams can hold, but also how much this single layer of OSB board can bear, and I would always rather be conservative there.

So if you want to use it permanently and the floor should be as level and vibration-free as possible, I would invest some money in expert advice and then sleep peacefully. Maybe it can be done like this (OSB + dry filling + dry screed + covering), but maybe the first layer has to be reinforced or replaced. It all depends on questions like how much the beams can hold and how much your first layer can support.

I don't quite understand the "waves" thing. The current panels are screwed in and strong enough to walk on, right? Maybe you can improve that by adding something underneath as needed or how/where are these waves? You could also screw another layer of OSB panels as a floor and then level them out at the necessary spots and lay them without waves. You will hear the "steps" underneath anyway; for me, impact sound insulation for noise reasons would be secondary in your project. I would stick painter's fleece on the OSB panels on the slope anyway and then paint them white/light.

The idea is not bad either; that's how we prepared the bathroom floor for tiling at our place. Double layer of OSB panel with some loose fill, then the rest. But that was a small room. In this case, there won't be heavy tiles but hopefully a lightweight covering. It would be worth considering, definitely the lightest solution regarding the weight.

I put a little water in the wine. In my opinion, the chipboard floor is a construction error. Below it, there is insulation that does get some moisture from below but cannot get rid of it because of the chipboard. This means there is a risk of mold. I advise removing the chipboard, nailing on tongue and groove boards, which are vapor permeable. On the tongue and groove boards, you lay carpet with a high natural fiber content. This way, you have a warm and cozy room. Karsten

I don’t quite understand the thing with the “waves.” The current boards are screwed down and strong enough to walk on, right? Maybe you could improve that by adding something underneath as needed, or how/where are these waves? You could also screw on another layer of OSB boards as flooring and then level them out where necessary and lay them without waves. You will hear the “steps” underneath anyway; for me, impact sound insulation would be secondary in terms of noise for your project. I would stick painter’s fleece on the OSB boards on the slope and then paint them white/light anyway.

First of all, many thanks for all the answers. By waves I mean that the boards, I assume they were seconds quality, and some are a bit bent, causing unevenness. The boards are firmly screwed down and walkable; we have lots of stuff stored up there that I’m now sorting through and want to throw away so there’s a base there first. My very first plan was to screw narrow battens onto the boards, then level the floor with filling material and slap on another layer of OSB and then laminate on top, but I think I need to have a structural engineer come to tell me how I can use this room regarding load-bearing capacity anyway. It wouldn’t help me to lay the floor and then not be allowed to put furniture like a desk or similar there.