They checked hundreds of bathrooms that were 10 years finished in Finland and many of the tiles had cracked when they had used Plywood, this is the system they now use in Finland.
Cut the plasterboard and lay it dry at a 45 degree angle to the tiles, leaving a 10mm gap around the walls and 5mm between plasterboards.
Then lift each plasterboard and put expanding foam under each board in the following manner; I put 6 S shapes of foam on each full sheet, two on the top, two in the middle and two on the bottom a line of foam around the perimiter.
Lie the sheet on the floor, put a screw in two corners,
Don't drive these screws home yet as they are only to hold the sheet in place while the Foam goes off. The foam levels off any inconsistencies in the floor.
When the foam is dry put in screws every two inches and cut away any foam that comes out between the joints.
Fill the joints between sheets with joint fill plaster and scrim, sand down and you have a perfect level smooth area for laying tiles.
We used a Scandinavian product from Mira to put a waterproof breather membrane under the tiles and to stop the tile adhesive moisture being sucked into the plasterboard. This gives better adhesion.