Take any random level 17-20 scenario and... just stop assuming the heroes are supposed to fight this thing, let alone win - and voilá, you have yourself a half-decent half-gonzo level 8 S&S scenario. Splash some mud, gore and supple flesh over the pristine four-color disneyfied presentation of modern D&D and you're good to go
From my experience running 5e Primeval Thule, this is exactly right.
Teens-level 5e in S&S is kinda ok but leans strongly towards the feel of the Marvel Savage Sword of Conan comics, with Conan as a superhero stomping on gods and Great Old Ones. The most fun we had was definiitely with level 3-4 PCs massively underlevelled for the adventure. Lots of "Flee, you fools!"
I did definitely find that with Xoth, running it in Dragonbane/BRP made for the pretty much perfect feel. Severos the Executioner getting his head bitten off by a baby Great Old One was a definite highlight.
