1% regret: https://pubmed.ncbi.nlm.nih.gov/33968550/
Meanwhile, finding actual studies of regret of cosmetic surgery is much harder, because nobody cares or want to stop people from getting boob jobs unless they're trans. So we're stuck with poor quality surveys like this one [1] claiming 65% regret, this one done for a group with a profit motive [2] claiming 65% regret.
For reconstruction after breast cancer, it's not "so bad", with this actual study [3] finding "only" ca 20% moderate to strong regret for even reconstruction, and another ~28% expressing mild regret.
Even for non-cosmetic surgery, such as e.g. hip replacements etc., it's hard to find regret rates as low as for sex reassignment.
Should we ban all of these other treatments too?
[1] https://www.dailymail.co.uk/femail/article-2640543/Two-third...
[2] https://www.medicalaccidentgroup.co.uk/news/do-you-regret-ha....
[3] https://www.researchgate.net/publication/233371110_Regret_as...