The thing is, OP, we know that if someone jumps out of a plane without a parachute there is an almost certain risk of death. So, no need to carry out RCTs on the efficacy of parachutes.
We also know a lot of things about the drugs which are used to block puberty.
We know that these drugs have been used to chemically castrate sex offenders as a form of punishment and risk management, where the physical side effects experienced by the person given the drugs have been either of secondary importance or not given any consideration at all.
We know that they have been used to treat certain aggressive forms of cancer, generally in patients who are already highly likely to die.
We know that they are used as sparingly as possible to treat women with endometriosis and children experiencing precocious puberty, based on a careful risk/benefit analysis and taking into account the known, serious, side effects.
We know that these drugs have a negative impact on brain and bone development, meaning that they can cause serious unwanted side effects such as osteoporosis and a significant drop in IQ.
So we know that they are not safe, we know that at least some of the side effects are not reversible, and we know that in practice they are not used to "pause" sexual development to give children time to think, because in almost 100% of cases those children go on to cross sex hormones as soon as they are able to.
Quite apart from the practical difficulties of carrying out proper clinical trials, arising from the fact that children will soon know whether they have been allocated to the test group or the control group, I fail to see how, given everything we already know about these drugs, it would even be ethical to carry out clinical trials, knowing what harm will most likely be done to children in the test group.
Also, as much as I hate to say it, it looks likely that healthcare systems in other countries such as the US will continue to hand out puberty blockers like sweeties, meaning that if we just wait a few years we will be able to see the effects of the widespread use of puberty blockers ourselves, without harming our own population.
Let's let those American doctors and hospitals with deep pockets pay out millions of dollars in compensation to people whose health they knowingly damaged for profit, rather than exposing our already overburdened NHS to inevitable clinical negligence claims in the future by continuing with "treatments" we have plenty of evidence to suggest are harmful and little to no evidence to suggest are beneficial.
In the meantime, it would be far kinder to explain to our dysphoric children that whether they take puberty blockers or not, there are no drugs or surgery in existence which will transform them into an adult of the opposite sex to the one they were born as, and they would be better off learning to love the bodies they are in rather than risking infertility, sexual dysfunction, osteoporosis and brain damage in a fruitless effort to achieve the impossible.