Whats the consensus on what’s a good time to start? I see a lot of talk on waiting, some people jumping into it from day one, etc.

My doc put it to me this way: if you start prog and e together you'll get more cone shaped boobs, if you stay on e only for a few years for your boobs to develop before adding prog you'll get rounder/fuller boobs after prog does it's work. I don't have references/papers that show this to be the case generally tho, just repeating what my doc told me before i started prog

From mostly hearsay that I seen and my recomandtion is when you reach tanner stage 3 and been on it at least a year.

There is not enugh research for any conclusions. Some started immediately and devolope round breast. Other cone shape. Now some develop cone shape even doing tanner stage 3 and a year part so that is even less fact.

Again there is not enugh research but I feel it is bad to gamble. Unless you find a certain feature from progesterone that you feel this important now.
Why not wait? You will have tons of bennificts from estrogen and effects from estrogen that is good that you can only point to estrogen instead having to figure out if it either progrestrone or estrogen causing them.

Especially with the very minuscule risk of backdoor dht with progesterone.

That definitely sounds like they're wither talking out of their ass, or had one or two patients Talk about their boob shape, and are drawing conclusions from a sample size incomprehensibly too small to be statistically significant.