Modern domesticated kings are different from a president trying to be king.

Americans hate kings because they were founded by revolutionaries who fought against their king. That revolutionary, republican (small r) spirit is foundational in American culture.

Preface: I don't dislike the King, I dislike the monarchy. Hate is too strong of a word -- I think it's not worth the effort to completely remove the monarchy from the Canadian constitution (it opens a can of worms), but I think next time the constitution gets changed it should be done.

Canada doesn’t pay anything towards the Royal family except the cost of visits

That seems like a great reason to not like the guy. He takes our money whenever he visits. If I visit another country they don't pay for anything.

He has no power, and he uses his influence

"Soft power" is another term for influence, so the king still has power.

campaign on the same things people on here like climate activism and access to healthcare.

That's great! Except he's doing it on private jets and with enough money to pay for many people's healthcare.

Honestly, I think Elon Musk has more credentials for political influence (he did at least run a few moderately successful companies with his parents' money). And I'm under no illusions that Elon Musk is a good influence. What are the king's credentials? Being heir to dictators responsible for colonialism, many wars, etc.? Sounds like a really bad case of nepotism. The king is doing good work now, but there's nothing stopping him from using his influence for something incredibly destructive in the next five seconds.

Political posting is mostly about jumping on a bandwagon, not any practical considerations. Adding nuance is no fun, so sure, they go straight for kill em' all, not "maybe relax the rules on taking oaths to the king". 99% will never do anything IRL anyway.