Best off ripping up the whole floor and relaying ply pva and then the ditra to avoid cracking

I would laytex the floor so it give you a good clean Base to start with and being a self leveling product the adhesive will bond to laytex better than laying on just tlies hope this helps

I suggest you to laying self levelling compound on top of the existing tiles.
Thanks

If you want the best results, I would remove ever bit of the old flooring and start new by self leveling then the heat matting if required and last the tile flooring.
I have seen it done the other way straight on top of the old tiling .

Tiling over existing tiles is not always the best option but if you have to for any reason then
First of all you need to make sure the existing old tiles are solid and not loose ,
If it’s solid and the sub floor got no movements
Then all you need is a double quote of a special primer to apply on the old tiles then you can lay what ever you need ,

a thin set of mortar is not a good option as it might become loose and causes cracks,

You can use self leveling compound ( laytex) to Level the floor before tiling if required,

If the sub floor is wood then you will need to use ditra matting to reduce the risk of having cracks in the future, but if the sub floor is concrete then I don’t see any needs for ditra matting,

Hope this will help you on taking your decision

Personally, I wouldnt lay anything on top of existing floor tiles whether they seem to be stuck fast or not. It only takes one tile to work loose and then youve got problems, ditra or no ditra. I would lift the old floor every time, not just for piece of mind but also to keep levels to other rooms as near as possible the same as it is now. For the sake of an hour or two's work lifting the old tiles is it worth the risk! And once those are lifted, chances are you can retile on what is already there, worst case some self leveling / thin set.

The use of levelling compound would be my choice. Its straightforward and leaves the surface ideal for tiling

It’s depends if there is height clearance for thresholds and under doors, but in my opinion If the floor looks sound enough, “no cracks to joints etc” I would grind it with a diamond wheel and dust extraction to give it key, and go with the ditra on top straight away.