Piece of shit. Simple and to the point. Shit’s pretty gross and it’s universal human instinct to avoid it. Maybe even shithead if you want a single word.

Asshole. Gender neutral and not tied to ethnicity since we all have one. Maybe even going further and calling someone an asshole related condition like prolapsed rectum or hemorrhoid, things that can happen to anyone, are pretty painful and definitely to be avoided, but AFAIK were never conditions that were heavily marginalized or shamed.

Clown or fool. Clowns/fools are types of characters people played historically (and still do?), IMO it doesn’t imply anything about a person’s actual intelligence or mental state, only their actions. You’re not born a clown or fool, but you can definitely act like one. Also lends itself to a snarky 🤡 emoji I’ve seen some people here use instead of typing out an insult.

Though there could be additional context or history to any of these terms I’m not aware of that push them into one of the -ist categories, IDK I’m not a linguist.