@cinnamongrammy for behind the toilet you might want to try cement board they use around fire place it’s thinner and you can paint it to look like drywall but is water resistant. At the gap between the drywall thickness and the cement board you can use some trim to hide it and make it showy if you want with with making the cement board higher significantly and using a nice crown moulding as a shelf hiding the seam.
Or you can use cement board and use the bathroom shower liner on top of it since water proof.