Levelling and prepping suspended timber subfloor

Getting ready to tile ensuite shower room floor and looking for a bit of direction on the subfloor prep.

The finish will be porcelain wood plank effect tiles (about 4sqm total) with under tile loose wire electric heating.

Room is first floor with 7" joists mostly 18" centres and original timber herringbones (joist span 2m max).
Old floor was trashed 1960's chipboard, full of holes and cutouts.
I have replaced floor with 18mm hardwood ply, primed all over, added a few noggins and screwed down at 150mm centres.

The floor slopes to one side and to the door with max 15mm from high to low.

Next I have 10mm insulated concrete tile backer board (warmup) to set and screw down.

QUESTION (at last): Do I use a suitable levelling compound or is 16mm too much on ply?
Could I use a thick bed adhesive under the insulation boards to even up the floor? The Mapei SuperFlex I used to bed the shower tray can be laid up to 15mm deep without shrinkage according to Mapei.
Could I build up the low half of the room with more ply or some 6mm tile backer i have lying around and then level out with either of the above two methods?

Although floor is already pretty stiff, my main concern is if that wedge of either SLC or Adi under one side of the room is going to deflect or expand differently to other side and leading to cracking.

 

Many SLCs like Weber Floor Flex allow 50mm in one go so shouldn't be a problem. Also use it to encapsulate the UFH before you tile on top.

 

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