When there is ambiguity in who holds power, there is conflict, instability, bloodshed, etc. This is why countries used to throw enormous festivals celebrating the birth of a monarch's first-born son — it means the succession plan has become entirely unambiguous, and stability has been ensured for years to come.
The public proclamations of the accession of Charles to King have included representatives of the state, military, police, and church. They have been made in public spaces. The mildest of protestations have resulted in arrests (albeit very few, and by seemingly confused police officers).
For me, the issues here are around obeisance and imposition. Apparently it's fine and "normal" for some elite [1] class of people to march into public spaces and make declarations that cannot be challenged.
The ornate outfits and ceremony are there to indicate a few things, the most obvious being that you are not one of them. In the strange post-modern Western world, pompous ceremonies come across as quaint and fun, another one of those bizarre things that the upper classes apparently get up to, silly them, aren't they funny hahaha. But it's not quaint, it's a public declaration that a tiny number of the ultra-priveleged still call the shots.
Bizarrely it seems that we lap it up. The message is almost subliminal now: you're there and we're here, and that's the way it's going to stay.
[1] I know the word elite is a dog-whistle but have you seen these dudes?? A more florid flaunting of elitism is hard to imagine.
If you dislike monarchies because of their power transition methods, I'd love to hear that reasoning.
The main thing you seem to criticise is the ritual itself, which to me seems pretty normal, albeit grandiose.