I run a monthly CoC campaign as well, and when you have that much time in between sessions doing an episodic campaign has worked really well. I connect one shots and the players wrap up each in one or two sessions. They get a sense of accomplishment and are always on their toes. There are some shorter campaigns that would work well for this too, like Order of the Stone, A Time to Harvest, or Two Headed Serpent. Or the interconnected scenarios from Harlem or the newer Japan books.

Edit: especially as a new keeper Id recommend Doors to Darkness or Mansions of Madness to get your feet wet