Bat's attacks don't have pieces of the carapace breaking off of Sage while Garou's does, even at parts that aren't the face.
Your version is implying a major weakness for SC that is never stated and is contradicted by the panels. If Sage's face has 6-C durability while his body had 6-A durability, why do parts of SC that are
not the face also crack? Why doesn't SC's face instantly gets destroyed by the attacks that broke his carapace? Why does Garou state "It doesn't matter what area I destroy, he'll just charge while regenerating"? Why does SC recoil the
entire head gets knocked back in response to Garou's attacks? How can Garou
crack SC's fingers from just breaking out of its grip? Why does Fighting Spirit + Resonance boost MB get knocked out from a punch from SC? Once again, Garou only brings up SC's regeneration as the issue, not its durability and he even scoffs at MB for being so weak and unable to deal any damage on SC. SC's reaction to Garou vs his reaction to MB even reflects that.
The scaling issues are easily solved even if your scale Garou to 6-A or whatever Sage's durability is decided to be.
Garou is 6-A, far higher with reactive evolution resonance, fighting spirit, and awakening breath.
Metal Bat never deals damage until after he gets an FS boost from SC's grand march that allowed him to easily destroy SC's tentacles. MB get's a 7-B, 6-A with FS, and far higher with Resonance.
Sage Centipede gets a static 6-A for damaging MB and Garou.
Sage Centipede and EOW's attacks may be calc'd to have 6-C or 6-B DC but that doesn't mean their AP is that low.