Also from the OP’s novel: I thought the chapter in The Grapes of Wrath with the tortoise attempting to cross the highway worked well as a stand-alone story.
The first book in Stephen King’s Dark Tower series, The Gunslinger, began life as a fix up of 5 previously published stories. The original version is more episodic, and I always liked The Slow Mutants as a stand-alone story. The revised edition makes the story more linear with a more pronounced through line and is more consistent with the later books.
Neil Gaiman’s “The Witch’s Headstone” chapter of his Young Adult novel The Graveyard Book has been published separately in at least two anthologies.
There is a chapter in House of Leaves (Mark Danielewski) where the children are talking to the father about echos in the house that could structurally stand on it’s own as a short horror story. I’d have to hunt it down and read it again to see if the dread comes across still not reading any of the earlier material.
One of my favorites is chapter 8 of MASH Goes To Maine.* It’s mostly a letter Hawkeye writes to the editor of the Maine Medical Journal, who had asked Hawkeye if he might be interested in writing an article about carcinoma of the trachea. Instead Hawkeye writes a story about an old family friend/patient he treated for carcinoma of the trachea. I’ve read the book twice, but I’ve reread that chapter many times.