Thank you @BonfireLady. I do really, really understand the desire for and appeal of a shiny, shiny, new, perfectly controlled study. I am a (retired) human behavioural researcher, I know what it’s like to look at all the shitty studies in your area and say, ffs, let’s just get one damn clean set of data.
But I can also confidently say that as soon as your study leaves the paper the proposal is written on, and hits the real world, you had better be prepared for real world issues. You either need to explicitly control for them ahead of time (as you point out, the PB study does not) or you need to learn how to do the type of research that can pull out information from noisy data (like the data that already exists from the Tavistock etc). The latter is harder, but people do that sort of research all the time, because sometimes that’s the only data you can get.
The people who wrote the PB trial, as you have laid out, do not seem to realise that humans are going to human. They have not written a study that is either remotely rigorous enough, or alternatively remotely flexible enough to accommodate that.
They also have not, as many PPs have said, identified their actual, falsifiable hypotheses. If you cannot say what gender identity is, or name precisely what improvement looks like - is it learning to live with their sexed body? Is it becoming more and more eager to surgically alter their sexed body? If it’s anything to do with improvement or maintenance of mental health, why PBs and not SSRIs? - then you do not actually have a rigorous study. Even the most loosey goosey survey study needs to have clear and falsifiable hypotheses - this study does not.