From my experiences with breast cancer consultants....
It's not so much the oestrogen as the progesterone. My consultant said this. (Women without wombs on oestrogen only have a lower risk of bc. But I don't know what happens if there's already an oestrogen positive breast cancer cell there.)
Higher doses of oestrogen require higher doses of progesterone. They have the data up to a 100 patch and equivalent. The synthetic progesterone is a known known with a very slightly raised risk. As with progesterone containing contraceptive pills - but the older you are the higher your risk of bc.
They don't have data for body identical progesterone iirc. Or it may be the same. But in my experience it can be a tricky one to use in hrt - some people find it makes them very groggy, some find it's not strong enough.
Women who have first pregnancies after the age of 30 are at slightly higher risk due to the hormones, as with any woman older than this and there's an increased risk within the first 10 years after a pregnancy. Due to the natural hormones.
Mirena is very effective- I wasn't allowed that even though it's a localised dose. Incase a stray cell is floating around and fancies settling in my bones and gets 'fed.'
Then there is also the issues around endometrial cancers and other hormonal cancers which can be impacted by too much oestrogen without the right progesterone level.
But this seems to have been lost in translation with the programme.
So lower doses can more safely use lower doses of progesterone.