There are surprisingly few Lovecraftian/Cthulhu ones. Oh sure, a lot modules throw in an element or two (and there's a love affair with frog gods and frog worshipers), but how many actually involve them in a major way? I can think of 2, that one from North Wind and mine about the Mi-Go.
The ASSH adventures are not the only ones leveraging the mythoi of Lovecraft, Smith, and Howard, but they are definitely the more-easily recognized ones. Kuntz's Bottle City includes a number of HPL-inspired gods, and his Lost City of the Elders/Garden of the Plantmaster and El Raja Key castle levels includes similar non-traditional play environs (less so the dungeons themselves; these two are available in the El Raja Key Archive). His Dark Druids leverages on Tharizdun (which is in turn based CAS' Thaisidon), but doesn't really tie-in the mythos much more in explicitly. Simliarly, Maure Castle has strong HPL-driven overtones (discussed in more depth in Kuntz's "Advent of the Elder Ones: Mythos vs. Man in the Lake Geneva Original Campaign, 1973-1976" in AFS#2).
Planar travel is largely untouched. I can think of a couple involving trips to Hell/the Abyss.
Have there been any OSR adventures focused on planar travel?
Planar OSR content is definitely a general gap, but are you looking for planar settings/adventures, or planar travel/mechanics, or both? Thus far, there have been a number of settings/adventures published over the years, but little in the way of planar travel/mechanics (outside of The Primal Order; for more on my distinction here, see my bibliography in the first link below).
To begin to fill those gaps, I wrote two long articles on "The Theory and Use of Gates in Campaign Dungeons" for Knockspell BITD at
https://grodog.blogspot.com/2017/05/the-theory-and-use-of-gates-in-campaign-dungeons.html and
https://grodog.blogspot.com/2017/05/the-theory-and-use-of-gates-in-campaigndungeons-part-2.html, and more-recently added another gate-related spell in The Twisting Stair #2---The Multi-Faceted Portal-Penetrating Gaze, a new 4th MU spell that allows you to scrye through to the other side of a gate before you decide to step through. I'm definitely working on more on this front, which the re-reading of Elric with my 10-year-old son has further invigorated (as well as our read-through of the World of Tiers a couple of years ago).
On the OSR planar settings/adventures front, Anthony Huso's A Fabled City of Brass is largely excellent---true to the spirit of the AD&D tropes for the City of Brass, but returning to its original inspirations in The Tales of 1001 Nights, while his Night Wolf Inn hearkens to a more-detailed and planarally-focused Comeback Inn from Blackmoor fame. Neither is really an adventure per se, but more of a setting/environ into which PCs might drop in and out over time as their quests take them to such destinations, or as waystops en route to other locations.
Allan.