cant speak to the rectal route, but prog seems to keep the t down and can play a complementary role to daily spiro if youre on weekly estradiol injections and want to reduce the risk of potassium buildup that might happen if you did high spiro doses instead. as for the feminizing effects of prog itself, it might help or it might not.
as for mental health, i find blood pressure to be the main difference ive noticed (with spiro specifically but also hrt more generally), but at normal female hormone levels the difference isnt that noticeable. there are peaks and troughs that go with the doses and schedule of anything you take