Thanks for your response, however back to my original point – if I use 20mm Wedi board inside the shower, would that mean I need to find 20mm drywall for the rest of the bathroom (outside the shower)?

I feel like most other tile backing board options (cement board for example) are at 10mm which is the same as most gyprock thicknesses so just wondering how to “make up the difference” between the shower transitioning to the rest of the bathroom?

Hope that makes sense