Floor structure in old building renovation bathroom

Hello everyone,
I am currently renovating my house and rebuilding the floors using OSB boards on wooden beams.
In the bathroom (12 sqm) and separate WC (1.3 sqm), I want to have the floors tiled. So far, the plan was to lay dry screed boards on the OSB boards and then tile on top. Now I do not want to unnecessarily increase the new floor construction height because the WC has a slope and every centimeter saved in height is helpful in the cramped space.
The question is therefore whether the double dry screed boards with 23 mm thickness are absolutely necessary, or if the single boards with 10 mm thickness are sufficient?
Or is it also sufficient to cover the OSB boards with WEDI Subliner Dry and Flex? That would, of course, be the optimal solution, at least in terms of floor height.
Thank you very much in advance for the help!
Regards, Roland

If you use OSB mid 18 mm with tongue and groove, a thin WEDI board is certainly a good construction

Thank you very much! However, the WEDI Subliner is not a board, but a membrane....

I did this years ago (bathroom renovation on wooden beams) and unfortunately failed to install a "black tub". Old buildings with wooden beams do not tolerate moisture in the wrong place. I would plan for it if at all possible. "Wedi" is also available as 4mm thin panels

Thank you! The corresponding sealing will be carried out in accordance with regulations. I don’t want to end up getting wet in the kitchen someday.