"Bill" wrote

2) Add more Durabond 90 to reduce the angle now, getting it almost flat,
and then tape it with additional all-purpose joint compound.

Almost. Tape, with 90, sor of following the jog with the tape. Then fill
almost level for at least a foot out onto the low board with 90 as the fill
material.

Then finish with all purpose to make more taper and refine the finish.

You should have asked about this one first. Adding some wedges on top of 2
x 4 and under the rock is the right way to handle that situation, even if it
means popping some nails loose on the old sheets to get the shims in. Too
late now.
--
Jim in NC