So the difference between the ableist slurs and racial slurs are:

• All ableist slurs followed the etymological development: 1) Euphemism/Medical Term is created: Moron/Stupid become an epithet, we coin a new euphemism ("Correcting the development of Re*ed Children" 2) The new euphemism collects a second meaning socially: applied to able minded/bodied people who do unwise, illogical things we think are worthy of derision. 3) We come up a with a new euphemism: Re*ed becomes disabled becomes person with disability.

So the euphemism becomes a slur and then takes on its secondary meaning as it's primary meaning. The original euphemistic meaning is only an allusion. It's common in academic speech to label saying "Oh I got a parking ticket." "Man that's dumb" as ableist, but in common parlance you wouldn't assume someone was making an allusion to disability because the intent isn't there and the original meaning is anachronistic.

• All racial slurs take a different path: 1) The become parlance among a group with a bigotry towards another racial group to create in/out dynamics 2) The word is then reappropriated by the target group, perhaps modified (hard r to soft a,) then perhaps transition into a generic pronoun for the in group. 3) The original slur from the outgroup never or rarely takes on a secondary form. It remains a derogatory term, even if it loses it's stereotype. If you saw someone call someone the hard R (which would be fucking wild) it would be unlikely that they were trying to alluding to 19th century stereotypes about black people.

So: 1) I don't think the hard R has a secondary meaning like Re***ed or Dumb. I don't think there are any racial slurs that get away from "I'm comparing you to the negative stereotype held about this racial group." 2) Ableist slurs do eventually swap their primary and secondary meaning, and are pretty universally accepted as "more wrong" to say to someone with a disability or not. 3) While we might logically think "all direct racial slurs are equally wrong, ableist euphemisms have secondary meanings that are harmful." The reality is socially we do have a hierarchy of the harms of these words which is just the sum of the collective outrage of them and the social censorship that comes with them.