Will this work and do I need a decoupling membrane

[tilerjon]

I'm sure the answer is here somewhere already and my apologies for asking this. I am, what I'd like to describe as, a competent DIYer and am now intending to create a wetroom in an existing first floor bathroom. It's a relatively small room, say about 2.5m x 2.5m. I want the smallest possible step in (or details as to how to deal with a larger step!) and was proposing the following.
Marmox showerstone former (22mm high)
Remove existing floorboards and replace with 18mm ply then 4mm Marmox TBB (which will bring me up to the 22mm of the shower former and give me a layer of insulation, albeit thin) with waterproof tape on all joins.
Then I'm stuck!
But ultimately, 10mm thick limestone mosaic floor tiles. Marmox says showerstone can take mosaics and this avoids having to put what I find to be ugly cuts in larger tiles.
So what about that "stuck" section. Do I need a decoupler such as Schluter DITRA Matting or can I tile straight onto the TBBs. And what if I wanted to add UFH would the order be Marmox TBB, UFH, decoupling membrane, tiles. Again, do I even need the Schluter.
Thanks for any help and again, apologies if the answer is on the site somewhere.

 

[Dan]

You tile straight to the backer boards.

I wouldn't have thought the ditra would be required with that sort of setup.

And if you had UFH it would be backer boards, ufh, self levelling compound, adhesive, tile. If you had ditra in that, it would be backer boards, ufh, slc, adhesive, ditra, adhesive, tile. So creating another few mm of height so as you've used new boarding to replace the floor boards, and used tile backer boards too, I'd imagine you'd be fine without ditra.