All three healers, if played optimally, revolve heavily around knowing the fight intimately and coordinating with your co-healer. That's not SCH's special territory at this point, especially given how lousy with DPS all three healers are. Which, yes, is another topic in the "why FFXIV healing is jank as hell", but it is what it is.
Trying to actually work out healing in any two healer pair comes down to working out who has what up when, how much can be healed through coasting on oGCDs alone, and if a GCD has to be spent healing, which of the pair loses the least doing it. Reactive healing has been bad for all the healers for a really, really long time. Trying to pitch SCH's strength as needing to know the fight makes no sense; that's true for everyone if they're doing it right. Coordinating Susano P1 for no GCD healing between a WHM and an AST is just as reliant on knowing the damage patterns as anything SCH has going on (and possibly slightly more so since the AST has to prep a big AOE heal at least 10 seconds in advance to abuse Stellar Explosion.)
Like, I totally agree that SCH/AST is a stronger endgame pair than AST/WHM. But AST/WHM is a stronger pair than SCH/WHM, because what you lose by not having a SCH at this point is super minimal compared to what you lose by not having an AST. AST/WHM may actually have a slight edge in prog for the average (read: not world prog) comp just due to WHM's ability to vent MP for years. Chain Stratagem--the big reason they're an auto-include in optimal at this point-- is also at the worst spot it's going to be at this expansion, given the widespread deemphasis on crit.
And it sucks in dungeons, which is killer. I'm leveling WHM last literally because it's the one I know I'll still have fun leveling; taking SCH into dungeons is just grim obligation.