How would you rewrite that part of the sentence that used the "R" word? It seems like "idiot" and "stupid" (edit: "moron") are also terms with historical baggage, and likely fall into the same category of being a slur. Or is it just because the "R" word is still in use by the medical field? I suppose "dumbass" is the first word that comes to mind without historical (or current?) baggage, and I'm personally fine to use that term to replace all the others above. But the links you provided don't really give guidance on how to express the above in a "safe" alternative way (note the original was an internal thought); I guess I'm here proposing that everyone just use "dumbass" (shout out to "Red").

Edit: I guess "dumb" also has its problems; perhaps even "fool"?