Raising a concrete floor by approx 2 inches.

Within a kitchen/diner extension I need to raise an existing conc floor approx 2 inches for two reasons:-

1. To reduce the internal cill height of two external doors, presently approx 4 inches..
2. To enable SWMBO to see the garden over the window cill when sitting at a table.
3. To enable me to bury / conceal CH pipes which have to pass in front of two external doors.

I am going to tile the floor afterwards and because of the drying time and mess of a wet screed, I'm inclined to lay a wood floor to bring it up by 2 inches approx and lay tiles using a good flex tile adhesive.

Should I use say 30mm bearers and perhaps 18 mm ply, What bearer spacing should I use to reduce flexing?

or should I use two or even 3 layers of ply (expensive?)

Should it be a floating floor or fixed (screwed to concrete).?

any recommendations please. Don