Disclaimer - I have accumulated a lot of knowledge over the years, but I AM NOT A DOCTOR
What is risky re blood-clots is estrogen itself - so I would guess that it matters little where it come from.
The major risk of clotting is when the estrogen is making it's first pass through the liver (it's route from the stomach to enter the bloodstream)
Pharmaceutical estrogen is a lot stronger and more effective than any plant-derived phyto hormone.
You could absorb half of a micronised estrogen pill sublingually (under the tongue) which is a direct route into the bloodstream, bypassing that first pass through the liver. (the average normal dose for a transsexualis 2 x 1milligram estrogen pills (Progynova) per day)
Alternatively, you might use Oestragel or Sandrena. These estrogen gels are applied to the skin of the inner thigh and so go direct into the bloodstream.
These are the lowest clotting risk and are given by British GIC's to older transsexuals or those with additional risk-factors as the safest option and are what I myself have always been on.
I have a low opinion of plant derived hormones which are very weak indeed and of unknown and variable strength.
I feel that if you want a weak dose of estrogen, then take a weak dose of ACTUAL estrogen.
- if I were in your situation and looking for mild, slow breast development, I would opt for a half of a 1mg sachet of Sandrena per day.