Drywall – What wall material should be used behind the Kerdi system in a tub/shower

You just want a stable platform for your tile. It depends on the size of the tile.. If you are cantilevering 10-15% of a larger tile, just fill the thinner areas with more mud and put backer where it will fit.

It looks like you needed some blocking behind the green board (where the rear curve comes in) to be able to fill in. Perhaps some fiberglass mesh tape would let you land some thinset to bridge the gap.

Since they're up, I agree adding blocking is a PITA.

However, if its an inside wall that you have access to the other side of, use a "BIG GAP" polyurethane foam. Drill a series of 1/4" holes every 12" down the middle of each joist bay. Spray a 3 sec burst in each. Wait 12 hr and try the flex, it should be quite firm now.

You may not want to do this near the plumbing valve, for replacement purposes. It won't hurt it, though.