Leave the old wooden floor for the bathroom without additional construction?

Thank you for the answers, it was just a thought! However, I see that we have no choice but to build a proper structure on the wooden floor in order to tile it. I wanted to avoid this, but this will create a small "step" at the entrance of the bathroom. But okay, we will have to and can live with that later.

If I have read correctly: Impact sound insulation on the original wooden floorboards, then 2 layers of OSB boards (glued together), then some kind of rubber layer for sealing (applied like paint), then tile adhesive and after that the tiles. Correct so far? I think this could end up being at least 7mm in height.

The most important thing is that the subfloor is rigid enough afterwards so that the tiles stay firm and do not crack due to the wooden floor giving way.

Hello questioner.
If the old floorboards are to be left in place without us later encountering conflicts with the build-up height, then that is fine.
A fleece underlay, basically as an "anti-squeak protection," is placed on the old floorboards, which must first be checked for a firm fit and, if necessary, re-screwed. However, this must not have a PE foil on the reverse side, so do not use any "painter's fleece"!
The whole system must remain vapor-permeable. Above all, before any subsequent work, check the flatness with a water level at least 2 meters long.
Even under load, nothing must bend!
On this fleece layer, then a layer of 22 mm OSB boards with tongue and groove (glue these well!!) is installed.
The entire area (OSB boards) is now coated twice with an alternative sealing (this is a suitable, two-component colored liquid plastic).
At the floor/wall transitions, a sealing tape must be securely and tightly integrated into this sealing. Pre-fabricated shaped parts are available for interior and exterior corners.
Definitely use a primer on the OSB boards (as a bonding bridge) and then bond the tiles with a flexible adhesive. Tiles on wooden substrates must not exceed the size of 30cm x 30cm. Otherwise, even with slight bending of the substrate, there is a risk of cracking.
This way, something will come out of the trade!
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Best regards and good luck: KlaRa

Thank you very much for your detailed explanation, KlaRa!

That is already a starting point, and we do without bathtubs in the bathrooms, which reduces the weight load in this regard. A full tub including material and cladding/tiles would easily weigh 250kg, which could cause the floorboards to give way.

What do you think of this Lux-Elements floor construction system?

Unfortunately, I can't tell you anything about the subfloor - only that I know terrazzo floors as quite common in old buildings for kitchens and bathrooms. Back then (in a house built in 1903) we had linoleum in the kitchen (and the retrofitted bathroom in the apartment) as well as terrazzo in the old toilet (on the "half staircase"). From a house built in the mid/late 1920s, I know terrazzo in the bathroom, kitchen, and pantry. If floor tiles are smaller in format than wall tiles, that doesn't look bad to me at all.

The special feature of our project: we want to install a bathroom in a room that was not previously a wet room, as mentioned the room has wooden floorboards on which the new floor structure for the bathroom is to be built.