How would you change Nocturnal AST to be competitive against SCH?

[_slowpoke_]

or.............. You could give WHM a chance to shine as the off healer.

WHM cannot "shine as an off-healer" because they don't have the MP solvency for sustained DPS in longer fights, you will simply run dry after just a few minutes without extensive outside assistance, and the same is mostly true for AST (unless you keep throwing Ewers on yourself, and that's the shittiest possible thing to do with an Ewer in raid content). That is a major part of why SCH is the dedicated off-healer and will continue to be - basically infinite MP while DPSing, there's no contest here.