I think Garling might actually be the arc villain, over Shamrock
Yeah if the Gorosei are all previous heads of the holy knights bestowed with immortality, it just wouldn’t make sense for the WG to leave it to the holy knights alone to beat the SHs + Elbaf warriors + allies, even if Shamrock is really strong.
Shamrock is calling MJ, probably for backup.
While it’s possible for Shamrock, as Shanks’s brother, to be a PK tier threat. I think Garling makes much more sense, as he has immortality now on top of likely already being a top tier beforehand.