I don't think it should be a scientific question, but rather a moral or ethical question, in which case it's very easy to answer. The focus on scientific validity mostly comes from religious transphobes who's main argument is God's design, which inherently needs to align with science despite how anti science some religious folks are.

At the end of the day the biggest scientific source imo is the statistic of people who benefit from transition and the lack of detransitioners. From a very basic scientific method approach, the hypothesis that trans people are happier from transition is clear to see. People that argue against that are scientifically/statistically wrong. Now that we know the proper treatment, this leads us back to the moral question of being trans.