Lmao! It's a good thing you've never read my comic. While it gets better later on, right now it's all "Wow, this sucks."
Honestly, as a reader, I don't really have a threshold. I can even handle really messed up rl stuff in portions. There are some things I will think are gross in a "that's unnecessary" kind of way, but in those cases I like to delve a bit into the person behind it. Half the time it's a kid, but sometimes you get particularly eccentric people who just like to make loud statements when arguing something. (It could also just be someone with a fetish or venting trauma.) At the end of the day it's none of my business, and I'm not particularly bothered by it, but it's still fun to get more context for particularly gnarly fiction.
Admittedly I am one of those types who seeks out disturbing content to push my threshold, but it's more of a control thing than a cute "Can I Stomach This?" challenge. Although it is fun to psychoanalyze the "artistic" choices behind the disturb stuff, sometimes you find really interesting things from delving into the deconstructive discussions on such media.
Speaking of, since you brought up 'A Serbian Film'. A while back I found a video and some discussion on the deconstruction of the movie, they had some interesting viewpoints, talking about how it's a visceral criticism of the porn industry. They also did analyses of the shot compositions, and what the director was trying to say with them. While I can't find it now because this was all before Youtube cracked down on mature videos, and the discussion was probably 10 years ago now, it did make me see the movie from a different perspective. Also, minus the gross stuff, I do think some of the shots look pretty good from a technical standpoint.
Not saying this to get you, or anyone, to watch it. I just saw the title drop and had to go on a tangent. Personally though, I think 'Salò, or the 120 Days of Sodom', 'Slaughtered Vomit Dolls' and 'Nekromantik' are more stomach churning. Saw those movies once and I'll be fine never seeing them again.
Now that I think about it the one theme I can't handle is upsetting pet related stuff that isn't cartoonishly over the top, and clearly involving stuffed "stunt doubles". Like, even the most vanilla "Pet got separated from owner and can't understand" stuff is too upsetting, although it's worse if you hear about inhumane behind the scenes practices. I used to love Milo and Otis, but upon hear about what they did to the cats and dogs to make the movie what it is, I can't bring myself to feel even remotely alright about it. (Thank you for destroying my childhood, Lindsay Ellis! x'D) Similar with hearing about messed up stuff happening to children and adults behind the scenes of a film, but I'm ashamed to admit that I'm particularly invested in pets.
As a creator it really depends on what I want to make. I like edgelord shock value stuff when that's the point of the story, I also like sexualized Grindhouse style of horror, and gross out for the sake of gross out. I guess if it doesn't fit in with the themes and context of the story then I won't add it in, also I probably won't ever be able to do dark, or remotely challenging non-andromorphic animal stuff.