Seems to me you just hit a potential plot point eh
Yeah, but that guarding should happen on Terra, in the palace, not as part of a "Legion", as the title would have it, and certainly not out in the galaxy. Even during the Beast Arises plotline they didn't do jack all to stop the orks, or bothered doing anything about the attack moon over Terra, even though it caused gravitational trouble. They only acted when a direct threat to the holy corpse manifested in the form of the Eldar Harlequins.
Their forces should remain massively diminished, not "legion". A resurgence of the Custodes in the galactic battleground wouldn't make sense, with the only guy they take orders from being, well, "dead". They're sticklers for the rules to the point of banging heads with Dorn and Corax, even when they were granted permission by the Emperor and Malcador when he was still alive. It doesn't make sense for them to venture out and do their own thing fighting xenos or traitors unless they are invading Terra itself, and even then only in close proximity to their golden father.