Canada

Now, as for actual discussion to be had, unfortunately our language is entirely coded in slights towards different groups of people. In calling someone “a sinister villain who’s a part of a cabal”, I’ve called them a left handed

if left handed people were still marginalised your comparison might be valid, but they aren't so it isn't.

At some point we do need to accept that these negative words, which are at their fundamentals, slights to certain groups of people, have taken on a new meaning, and that their misuse as slights against those people only really applies contextually.

No we don't, especially since no, they haven't

I do think that terms like “stupid” and “idiot” have achieved that level of shift.

you would be wrong
https://thebodyisnotanapology.com/magazine/stupid-is-an-ableist-slur-breaking-down-defenses-around-ableist-language-liberating-our-words/

I’m not here to tell you you or your experience is wrong,

yet that's exactly what you are doing.

You should be able to remove a few words from your vocabulary to reduce harm to already marginalised people, without arguing about it or trying to "logic" your way out of it. It isn't a big ask, but rather the very bare minimum.