I think you are right that Faulkner is not an easy writer to get to know. I make a few remarks about Faulkner in the Reading List for American Modernist Literature that you can take a look at, for a start: https://readgreatliterature.com/literature-lists-timelines/literature-american-modernist-literature-1915-1945/.
Faulkner is difficult mostly because of his technical fireworks. Once a reader learns to navigate those, the themes become more apparent. In fact, oftentimes the technique IS the theme or idea. Let me see if I can explain:
Faulkner’s preoccupation was with the way we tell stories about ourselves and others. We receive judgments, values, and narratives from families, culture, or local history, and often apply or enact them in warped ways. Also, people are naturally fascinated by other people, but because they can see only a part of what others think and feel, they make up a lot of stories about others and don’t realize they are not seeing the exact truth.
Further, people recruit other people to play roles in their own false personal dramas, and thus end up exploiting them, whether to a large or small degree. People even tell false stories about themselves that they proceed enact, often to the detriment of themselves and others. Thus the bad guys in Faulkner use others, either as characters in the stories they spin, or actively exploit them for personal gain, without regard to that person’s inner life or needs; the good guys (the few there are) have more empathy for others, allowing or encouraging them to express their own identities. Thus as a writer, he tries to enact that kind of empathy, spending a lot of time inside people’s heads to try to render how they think and feel, and discern the kinds of cultural forces and experiences that made them that way.
Faulkner also critiqued much of small-town Southern culture. His stories illustrate how racism, privileging blood ties over individuals or moral principles, and too much adherence to a medieval-esque code of honor all warp people, damaging their identities and causing grief all around. Blind personal ambition is often a villain as well.
It sounds like you have read a lot of Faulkner, but to get more comfortable with his technique, I would recommend starting again with a couple of the short stories, “Barn Burning” and “A Rose for Emily,” reading not just for theme but to analyze his writing techniques. Check out my post on “Rose for Emily” for a discussion of both technique and themes in that story here: https://readgreatliterature.com/literary-acrobatics-william-faulkners-rose-for-emily/. In “Barn Burning,” notice how Faulkner keeps our perspective mostly inside the head of the small boy, with some outside commentary from the narrator, to illustrate how he struggles with divided loyalty between his father (the blood) and his keen sense of morality. Family or Empathy? Loyalty or Morality? Sarty must choose. This is a small casebook for Faulkner’s major themes.
From there, move on to Light in August. This is easier to read than some of the more famous ones, but does still use some flashy writing, such as stop-action scenes, and other scenes where we stay inside the perspective of a particular character. The book is about how various sicknesses in the older Southern culture damage the characters and encourage them to damage others. But the book also offers the way out of this cyclical misery: an imperfect “romance” between the pregnant waif Lena and the kindly Byron. To fix a broken culture, just have a kid and start over. Life goes on when encouraged by kindness. He also illustrates that enlightenment is possible, as the Rev. Hightower has an epiphany at the end that his cherished code of honor was actually inhumane.
After that, try As I Lay Dying. Here Faulkner examines the range of psyches found among the stubborn poor. What motivates each individual to hang on or a pretty useless quest? One aspect not to miss with Faulkner’s work is the humor–rather dark, of course, but often sad-funny, not just sad. There’s humor in this work along with angst. Then try The Sound and the Fury–true stream of consciousness, shifting from the mind of one character to another. If it is hard to keep up with the changing perspectives, take a look at sparknotes.com or another reader’s guide to clue you in. The themes in this work are similar to Faulkner’s other works. One of the cultural problems that haunts the characters is the way all the male characters are preoccupied with the honor of their sister. Sounds like a good thing, but the story shows how the whole family is warped by this preoccupation.
I hope that gets you started! Let me know how it goes.