Flooring after renovation - Recommendations from the landlord's perspective?

Well, the original floorboards were only 22mm. I was thinking of 27mm or 28.5mm boards. What kind of fill would you recommend? I also removed the old boards of the crawl space. Could I use 16mm chipboard instead, or is that not stable enough?

They will bend. And with a joist spacing of 85 cm, the material thickness should be at least 35 mm. And does the floor need to be leveled? How do you get the idea to just tear everything out without a plan?

The attic has only 1.90m standing height in the middle. A large dormer increases the usable value somewhat.

Is your upper floor also the attic? Then you don’t have any official living rooms upstairs, except probably those by the dormer.
As a landlord, you should calculate the official living area.
Small apartments made ready for couples are often not lived in long. Therefore, you have to weigh whether to lure them with the equipment or wwi.
You have to do some research yourself.

Of course, it's a lot of work to get all the loose fill out. But the good lady urinated there. The rats (which looked quite puzzled during the clearing out) were also not house-trained. Oh, and there were one or two cats there as well. The smell was intense, and I think enough has seeped through the floorboards into the fill! Ozone sounds interesting, but this feces simply ran in there, so everything had to be removed. I definitely wouldn’t want to let anyone move in there. That’s about this operation. I now want to pour in an alternative product. Perlite seems too light then. Blähungen? How about that? Or is simple sand enough? Yes, the topic of old building renovation is full of half-knowledge. That’s why I shared my thoughts with you beforehand and hope for factual advice. Thanks again

I did not question the necessity of renovating the floors, but was surprised that you focus so much on which covering offers the best cost-benefit ratio for landlords. Perlite is a good option as a new filling material if you don't want to add too much weight.

Topic flooring: I myself like wooden floors - whether plank flooring or parquet. Plank flooring is interesting because it is screwed down and at the same time constitutes the finished floor. The detour via OSB/company Ellis + impact sound insulation + parquet is simply also expensive (at least double the price compared to plank flooring). And since it is actually only intended for rental, I would like to learn the thoughts of experienced landlords on how they would weigh it up. Thank you