Shower niche in 2x4 wall with drywall back, can I waterproof back of drywall with redguard+fabric?

I’m trying to maximize the depth of the shower niche I am building and would like to know if I can leave just the back side of the niche which is the brown side of drywall.

The walls of the shower will be covered in wonderboard and I’ll frame of the sides of the niche with 2x4s and wonderboard but instead of adding a piece in the back do you think I could get away with it going directly to drywall?

To waterproof I’ll be applying redgard with laticrete membrane fabric over seems and could cover up the back wall with overlapping layers of the fabric as a paint on the redgard in addition to the seams in the corner.

Would caulking the corners between the drywall 2×4 connection be a good idea too with a product like kerdi-fix?