Floor Boards / Chip Board & UFH?

[2020Tiling]

Hi,
Need some advise on a Bathroom floor I'm starting next week.
Floor is half Floor boards and half chip board which is around 10mm lower than floor boards.
Floor has no bounce.
Tiles are 800 square Porcelain and customer has UFH Mats.

Was thinking

Hardy 6mm over whole floor
SLC to make flush
Ditra crack matt
SLC
UFH?
SLC?
Tile?
Just wondering what method you would use & would you fit insulation boards ?

 

[White Room]

If the chipboard floor is lower than floor boards of 22mm thick, I'd be checking the thickness of the chipboard floor.....