I actually recently decided to wait until post-SRS or post-Orchi to keep taking progesterone. I tried using bioidentical P cream for a couple of months, and I started seeing androgenization effects... hairs sprouting back up, sex drive went back up, looking less feminine to myself. And I read a study somewhere that when exposed to testicular tissue, progesterone converts to testosterone, so I got scared and gave it up.
And technically, most of female puberty takes place in absence of progesterone... progesterone doesn't come around at all until the onset of menses, which is usually 2 years or so after beginning puberty, so it makes biological sense to wait a bit as well.
Not sure if any of this is accurate or not, but just something to consider.