Interesting topic OP - thanks!
Just to state my position - I'm a mild constitutional monarchist for many boring reasons I won't go into here.
Yes, by a democratic vote - but how that might come about is anyone's guess. An MP or the ruling party at the time would have to initiate the process and presumably campaign for public support.
The mention of Jamaica is really interesting. The Queen always said that she was there to serve the Dominions (Canada and Australia) as Head of State as long as they wanted her - and when they didn't, that would be fine and everyone could remain friends. She loved the Commonwealth and was very keen to keep it going - and I think when it was started, it was a force for good and friendly relations between the UK and those newly-independent countries. Again, the line from the monarchy was always 'We hope you'll want to stay, but no hard feelings if you don't'.
Whatever you think of the concept of the Commonwealth, what is happening in places like Jamaica is actually worrying, I think. These countries - which are on the verge of demanding independence - are being targeted by China with lots of infrastructure investment etc. It's obvious why this is attractive to developing countries, but the quiet take-over by China is sinister and is already ringing alarm bells.
My personal opinion is that the Commonwealth has served its purpose and, with the Queen's passing, should now be allowed to quietly fade away. But to put the other side of the argument - a Commonwealth head did once say that it gives very small countries a place at the table they wouldn't otherwise get, and that they valued that.
But I think, whatever your political opinion, what is happening with the RF just now is not good news for any of us. Whatever you thing of the monarchy, it's been a stablising influence in the UK and the world for a long time, and the fact that it's currently wobbling, on top of all our other troubles - and nightmare scenarios in Ukraine and Israel/Gaza, a pair of duds competing once more for the POTUS - isn't something to celebrate. Hostile powers will rub their hands in glee and surely try to exploit the situation.