The Ascension enthrones human nature in heaven, a change in what humanity is. The Annunciation is where that nature is assumed, but it is narrated as promise and consent, not yet accomplishment.
If the Incarnation begins at conception then the Annunciation is Christmas's actual content, nine months early. The Transfiguration only discloses what is already true.
Theophany is a manifestation of the whole Trinity at once. The Annunciation is an announcement to one person, however consequential.
An event in the economy of salvation over a commemoration of doctrine, even a foundational one.
Conception over instrument. The Cross matters because of who was on it, which is settled at the Annunciation.