An interesting tidbit is that the feast of the Annunciation on March 25th is actually quite a bit older than that of the Nativity.
The Nativity of Christ comes exactly 9 months later, on Dec 25th.
Thus, there is a strong case to be made that while Christmas did displace the "sol invictus" pagan feast, and coincides with the general time of the winter solstice, there may be ancient traditions within the Christian world that preserved the memory that the Archangel Gabriel visited the Virgin on or around March 25th.
Not a thing that is likely to be forgotten...
My mother, of blessed memory, always denied that Christmas was on December 25 because it replaced the Saturnalia. She pointed out what you just posted, namely that the Feast of the Annunciation had been celebrated even before Christmas and always on March 25. The calculation is easy to make. She said that her grandmother had told her this and she had heard it from hers and on back through the centuries.
Whether March 25 predates the absorption of pagan holidays doesn't counter the point that I made: That many parts of the liturgical calendar are a result of Christianizing what were pagan celebrations. Why not continue to do the same, and make so that our feasting and fasting days are congruent with the how our cultures holidays have evolved. We can re-Christianize it again.