So your success metrics are also vague and difficult or impossible to measure or define? How do you define "long term happiness" and how do you propose that could ever be separately identified and traced back to a single cause without any other factor in that person's life affecting it? How would you measure it?
If these are your success metrics then presumably you accept that this experimental medical intervention is attempting to treat a mental illness rather than to remedy a physical malady?
Do you think these are the same success metrics used to gauge success of other medical interventions?
I am not sure if it is your intention but you do appear to be defending medical experimentation for the purpose of "treating" an indefinable condition in order to achieve indefinable outcomes. Nobody can define what trans is and evidently nobody can define what a successful transition is either.
I am also confused by your "decreased mortality" metric. The myth of increased suicide rates has already been exhaustively debunked by credible experts, so I am not sure of the relevance of this proposed metric.