> This actually terrified me, as Britain being willing to give up the commonwealth when the Queen dies may mean serious contests for who gets to write new constitutions
No one is talking about terminating the Commonwealth. Nor does Britain qua Britain have any role in any of this. These are separate constitutional monarchies vested in the same individual but with entirely separate governments. Britain already gave up authority over them when they became independent.
> It's not so much a matter of removing the Queen as head of state as unsealing the document
In many cases, the documents have been revised multiple times in recent decades. They aren't Holy Writ. And while a less popular monarch may accelerate Republicanism, its unlikely to trigger a simultaneous rush for the exits.
> someone says while you are there, we need this, and we will fight you if we don't get it. You see how this escalates immediately, and all together within months of the Queen's death.
I see what you arr imagining, but I don't see it as particularly likely.
> What are the odds of over 50 countries ever trying to rewrite their constitutions in a lifetime,
Pretty high. In fact, pretty much every country on Earth, of which therr are significantly more than 50, has revised their Constituion in living memory, most have done so in the last 20 years. [0]
> and then again what are they to do that simultaneously ?
Again, there are probably more than 50 working on revisions at any given time. Not unusual at all.
> I can only hope it will be mostly peaceful?
Why wouldn't it be? The (much fewer than 50) countries sharing a monarch with the UK are largely ones that have revised their Constitutions peacefully multiple times recently.
[0] https://comparativeconstitutionsproject.org/chronology/