Wet area floor former underfloor heating

[silver50]

Hi folks, regarding tiling the sloped floor former and electric underfloor heating. Having just laid heat mat in my main bathroom I feel aware of how much it raises levels. I don’t want to upset the floor former’s slopes (not too bothered about trying to mat over it..) but I’d quite like to mat the flat section going towards the steep slope (above stairs). I feel for one it would be good help dry a “wet area”.

As tilers, what do you advise? Should matting almost the whole area be ok, or would you stick with my theory of keeping away from the floor former itself?

On tiling, if I do it myself, are smaller mosaic type tiles a good idea to save trying to cut diagonals etc on the floor former? What would be a good choice for the more amateur tiler?