Threshold Strip into tiled bathroom

Hi all,
I am just about to begin tiling my bathroom and am concerned about the difference in level through the doorway.
The tiles are 9mm thick, I am told I should put down some 6mm ply over my existing floorboards to add stability and also to expect the adhesive to add 4-5mm of depth once laid. This will give approx 20mm of depth to the entire floor which seems quite a step for a threshold strip. What does anyone advise? Have you any recommendations?

Regards
Steve

There are flexi adhesives and grouts available that can go straight over T&G floor boards - no underlayment necessary. Call Ardex or F. Ball.

FWIW: adhesive in typical situations can be less than 5mm. Use a 3mm notched trowel, & a tight buttering to the tile back using the trowel flat edge.
6mm ply is a waste of time in my view - it can swell & buckle, and pull the screws, when moisture from the adhesive soaks the top face. Go 12mm ply or not at all. Just my view.
6mm backer board is OK though if its screwed & glued.

Ply needs to be 12mm minimum, though you could use thinner backer board, the threshold is never a real problem only perceived to be, make sure tile edge is tight to suitable threshold to avoid chipping the tile edge.

9mm or 12mm ply isn't going to add much stability to the floor. If it flexes then you'd need to fix that first.

If you need to overboard then 6mm tile backer board will be better than ply but it won't stop floor bounce so fix that first.

Don't worry if the floor finishes at a different height - it's east to fit a cover strip to deal with the difference.