Law's injection shot made Kaido bleed (which means he doesn't need Awakening to make Kaido bleed)
so if Kaido's durability > Big Mom then he could have just used injection shot and made Big Mom bleed more as there was no need for him to use moves which consumes extreme amounts of stamina
This is first awakening move. He used it because he is going to use his entire arsenal now and thats gonna be more devastating. That is the reason why he uses these moves even though they take away stamina.
The thing is Injection Shot and Anesthesia did roughly the same amount of damage. Despite both working on the same principles to circumvent durability.
Gamma Knife doesnt bypass durability as show against Dragon Kaido. That doesnt make Gamma Knife weaker than these two moves since the durability of Bm and Kaido prevent it from damaging them.
Kinemon's attack damaged Kaido while Wano Act 1 Luffy's G4 moves didnt. That doesnt make Kinemon's attacker more stronger. Its just that G4 lacked the Ryou to bypass durability.
Injection Shot, Anesthesia and GK when used against characters who dont have such durability defense would provide a better way to scale. IS and Anesthesia would do the same amount of damage while GK would kill that character.
A > B > C doesnt apply here.