ᐅ Floor structure in bathroom renovation of an older building

Hello everyone,
I am currently renovating my house and rebuilding the floors using OSB panels on wooden joists.
In the bathroom (12 sqm (130 sq ft)) and separate toilet (1.3 sqm (14 sq ft)), I want the floors to be tiled. So far, the plan was to lay lightweight screed boards on the OSB panels and then tile on top. Now, I would prefer not to increase the floor height unnecessarily because the toilet has a slope and every centimeter (inch) saved in height is helpful in this confined space.
My question is whether the double lightweight screed boards with 23 mm (1 inch) thickness are absolutely necessary, or if single boards with 10 mm (0.4 inch) thickness would be sufficient?
Or would simply covering the OSB panels with WEDI Subliner Dry and Flex be adequate? That would be the best solution in terms of minimizing the floor height.
Thank you very much in advance for your help!
Best regards, Roland

If you use OSB panel 18mm (3/4 inch) thick with tongue and groove, a thin WEDI board is definitely a good construction.

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

I did this years ago (bathroom renovation on wooden beams) and unfortunately forgot to install a "black tank" waterproofing layer. Older buildings with wooden beams cannot tolerate moisture in the wrong places. I would definitely include this in the planning if at all possible.
"Wedi" also offers panels as thin as 4mm (0.16 inches).

Thank you! The appropriate waterproofing will be carried out according to regulations. I definitely don’t want to end up with a wet kitchen someday.