In extreme circumstances if Charles really can’t carry out his duties he could declare a regency
No he couldn't. Parliament would need to legislate. (There probably is a Regency Act on the books, but it might need updating for the specific circumtances.)
The whole point of a regency is that the sovereign is incapacitated. If he's incapacitated, he can't make declarations about anything.
the monarch isn’t essential enough these days to need a Regency - other royals would just step up to cover royal duties.
We're not talking about visiting hospitals and opening things. This is about the sovereign's constitutional role. At present, if the sovereign cannot act, two counsellors of state are needed to act in his/her place.
The day before she died, the late Queen cancelled a meeting of the Privy Council. It's thought the then Prince of Wales initially sent for William not because it was thought the Queen's death was imminent, but in order to hold the PC meeting, which needed two of them.