Wet room flooring dilemma

So it was a toss up to where i should post this - landed in plumbing with the hope to get bathroom fitters attention.

So i am about to rip my current main bathroom out as part of a larger renovation project we are undertaking at home and turn that into our en-suite wetroom.

Current floor is 18mm softwood T&G with 2mm ply pined down, plan was to rip that ply up, fire down some 6mm tile backer, UFH, SLC then tile. BUT the Impey Aqua Dec is 22MM and they say the dec has to be perfectly flush with the floor around it.

What do i do?

If i put the backer board down to the dec then were 2mm Proud before UFH and SLC, will i loose that 2mm?

I planned on running the UFH into the dec so i was going for the adhesive mat rather than loose wire, the Impey membrane acts as a decoupler so saves another layer, but again, how far into the dec do i go with the UFH to the drain.

Any advise will be HUGELY appreciated

OR, i could rip the floorboards up and put down (dont judge!) 22mm green sh1t chipboard glued and screwed, the floor will be fully tanked so unlikely to ever see liquid.

have you worked out a rough figure for what the real running costs will be for any kind of UFH in your bathroom?

Only what is given to us off the websites, which is pretty much sod all in the grand scheme of things, we only want it as a secondary heat source to keep the tiles from being cold.

why do you say that anyhow.

2mm is fine and it's better that the rest of the floor is proud of the Dec.
This way, you only have to make up 2mm with tile adhesive where the entrance to the Dec is. The side will rise above anyway, as the Dec slopes down to the drain.

Bring you UFH into the Dec just up to the point where you will be standing. You don't have to go beyond the drain.