Bathroom floor - ply subfloor under insulated backer board, or insulated backer board direct to floorboards?

Renovating our 30s semi bathroom soon. Planning on tiling the floor onto an insulated tile backer board from WEDI or similar, to hopefully help the tiles feel a bit warmer underfoot during the winter.

Is there a need for a ply subfloor or can the WEDI insulated backer board go direct onto floorboards? In an ideal world I would screw ply down first, then fix the WEDI boards down to the ply with tile adhesive before finally tiling. This would achieve rigidity and strength from the ply, and waterproof/thermal benefits from the WEDI, while providing the perfect tile substrate. However, is this overkill? This is likely to build up the floor thickness considerably, which may make transitioning to the carpeted hallway difficult or look a bit weird.

On the WEDI website here https://www.wedi.net/uk/area-of-application/interior-building-board-applications/floor-applications/wooden-substrates/ they appear to be fixing the boards directly onto floorboards, but I'm not 100 percent convinced that 30s floorboards will be meeting the conditions "The substrate should be rigid (not vibrating or sprung), be able to withstand the load, be dry, free of dirt and damaging infestation. Any unevenness, opening or crack must be levelled out."

Thanks once again DIYUK hive mind!