In a North American context*, "Karen" is a specifically misogynist, sexually violent, rape apologist, and anti-black racist slur. Anyone who still uses it unironically is at best wilfully ignorant, and most likely misogynist, rape apologist, and white supremacist.
The term started with a seventeen year old white boy in California who started a forum (AKA a "subreddit") on reddit.com called r/fuckyoukaren asking members to contribute their suggestions about how best to sexually abuse, punish, and humiliate his (fictional) "ex-wife", Karen. The sub is still there; you can go and read it yourself. If you doubt this, go on reddit and do a search for "Karen". If you still endorse use of the term afterwards, you can tell your wife or sister or daughter and/or every other woman you know why she deserves sexual violence simply because she is female.
I can't specifically blame the child who started r/fuckyoukaren for this, but at some point black women in the USA were vociferously blamed for "Karen" as a term. It never was a black American term; misogyists and anti-black racists made it up to divide black and white feminists. This is a misogynist rape-apologist, rape-promoting trope and every dud here defending it is a racist misogynist. And yes, if that's you, that means you're a bigot.
As far as whether we should add Keith or Kevin or Gammon (is that a name used for boys in the UK?) ... hell. no. I don't want men to be dehumanised and subjected to threats of sexual violence either. What kind of nihilistic arsehole does?
(* If "Karen" has a meaning in the UK which pre-dates and is independent from the North American meaning, then please post some evidence of that.)