Ceiling between ground floor and first floor has sagged/leveling

So no filling between the beams and also no dry screed panels but rather use OSB boards?

Unfortunately, I am maximally limited in the build-up height. The room was already quite low before. Therefore, I have to pay attention to the height.

Good evening,

unfortunately, I have to bother you again.

I have now evened out the beams by attaching slats on the side. As suggested, I will fill the gaps with insulation for soundproofing and lay OSB boards on the evened out beams.

For stiffness, I was thinking of 30 mm OSB boards with tongue and groove. Is that too much, or do you rather recommend 2x 15 mm OSB boards installed offset, or a thinner thickness of OSB boards? Should the boards then be laid floating or screwed to the slats?

Another question: One room is to become a bathroom where large tiles will be installed. Should I omit the OSB boards there and instead lay dry screed, use a combination of dry screed on OSB, or are the 30 mm OSB boards stiff enough to prevent cracks?

The beam spacing is about 60 cm. By attaching the compensating slats, the spacing has naturally decreased. Despite everything, I am now completely at a loss, as everyone recommends something different. I will probably accept flexible adhesive and a fabric mesh for the tiles. But is that sufficient?

Use OSB boards, dry screed with a span of 60 cm will be botched. And otherwise, if everything is open, clarify what the cause is. It could also be that beam ends are moldy, pests, etc.