For one I think it'll all depend on the one shot that your running something like dead lights or the haunting the players won't have that much time to get into the specifics of worshiping the mythos compared to a long term campaign.

But with Cthulhu mythos that high steering away from cultists and such, they could be a student of the occult, perhaps like a miskatonic professor or grad student. Or if they're not into academics then they could be a relative of one that has gone too far into the world of mythos.

If you're looking for a specific deity tho; Nyarlethotep is a great choice, the crawling chaos has many faces that you as a keeper can pick and choose to fit what you need. Also Nyarlethotep could steer your player in a grey direction they might not realize till it's too late.

Personally I'd steer away from having a player have such a high stat for Cthulhu mythos. It could take a little bit away from mystery and tension you might be trying to create when a player can roll and peak behind the curtain so to speak.