Rappan Athuk. Have the party enter RA through the Cloister of the Frog God (Tier 2) or the Cavern of the Kraken (Tier 3).
Using bits of Rappan Athuk sounds plausible - I've just come off a couple years running Stonehell megadungeon so not looking to do the whole thing.
I'm really looking more for specifically Conanesque swords & sorcery fantasy rather than 'D&D fantasy' (even dark D&D fantasy) though. So closer to pulp action-horror adventure stories; the kind of thing Conan & co might have stumbled across.
Looking at my resources, there are a ton of low level S&S themed adventures that could be converted; I think stuff like The Licheway & Halls of Tizun Thane from Best of White Dwarf just need their levels bumped up a bit, pretty easily done just by making goblins into Tcho-Tchos, lizardmen into Deep Ones etc. I had been hoping for specifically 5e D&D S&S but that isn't looking too promising.