If there was a way to make sure they're not going to change their minds later then starting with the treatment as early as possible would seems like the obvious way to go, but it's not quite that simple.

Except that's fine if they do? If you stop taking puberty blockers, you start going through puberty. This is not some magic irreversible medication or something, for the ~1% that do change their mind this isn't an issue.

I could maybe see this argument work for gender reassignment surgery, but not for puberty blockers.

We should let healthcare experts make healthcare decisions.