I would just get a standard vaginal estradiol 0.1% cream and use it daily externally in addition to the prasterone.
Systemic absorption is negligible, however when genital tissues have gone untreated for a prolonged period they can become so dessicated and the skin barrier so dysfunctional that paradoxically there is an initial, transient increase in systemic absorption (still clinically minuscule) until the health of the tissues has improved.
Additionally, the nocebo effect is a very real, scientifically established phenomenon, I have experienced it myself in a different context, and I would not be surprised if following your cancer it had been absolutely hammered into you that estrogen = the devil. Therefore feeling potentially stressed and conflicted about using a vaginal treatment could quite plausibly have exacerbated your migraines.