• puberty blockers, including their side effects, in order to make the rest of the transition later on (with or without surgery) easier
  or
• no puberty blockers, avoiding the side effects and making a potential retransition later in life easier, but at the cost of making them miserable throughout puberty and making future stages of the transition more difficult

And sure, the second option is 'doing nothing', but doing nothing is also a choice, which has upsides and downsides too.

Your position is that if this teenager chooses the second option, of their own volition, then great; if they choose the first option, well, that shouldn't be allowed because of the downsides involved. So let's just take their choice away and only leave the second option. But that second option has downsides too, in fact worse ones at least in the short term and possibly also in the longer term, while its upsides are quite uncertain since they only apply in the event of the teenager wanting to revert to their birth gender later on.

Out of curiosity, do you think teenagers should get to make their own decisions on other difficult medical questions, like for instance in a case of advanced cancer with a choice between a final attempt at chemo which has a low chance of success but is guaranteed to make the patient's life a lot more unpleasant in the meantime, or giving up and letting the patient spend the final days/weeks of their life saying goodbye in comparative calm and comfort? That's not intended as a full analogy or anything, the choice is quite different, but it's also a case of a difficult medical choice, between 'doing something' and 'doing nothing', where you definitely want to be making an actual conscious choice and not just walking into something by default because you're too afraid, or just not allowed, to make the choice.