During a pre-conflict action window, Way of the Phoenix is played by the non-active player, choosing the Void Ring.

Can Isawa Kaede be declared as an attacker that phase, even if the attacking player declares the Fire ring for the conflict?
Are you considered to be declaring a Void conflict, as well as Fire?
At what point does the ring gain the Void element?

No.

After Way of the Phoenix has been played on the Void Ring, the opponent cannot declare Void conflicts. As all declarations for a conflict are made at once (province, element, characters, type), declaring a conflict with Kaede always counts as declaring a Void conflict (and Fire, in your example). She adds the element whenever she is participating, which also includes when she is declared as an attacker.

[Tyler Parrott, Apr 16 2019]