This point about the ECHR is missing what Straw’s expertise is here and shouldn’t be ignored. He implemented the ECHR into UK national law via the HRA. Now I don’t think he’s wrong. You can provide a nearly the same guarantee to the GFA and Ireland. That is legally possible. And frankly, if the alternative is a British government that is Reform, you might think it quite sensible to protect the effect.
I read the Policy Exchange report. It seemed me to pretty credible. The objections are not legal but political. Politics changes. Do not think the GFA immutable. Or any treaty where the UK is a party. History tells that is completely wrong.