When spires fire, do I choose the targets for all of my spires at once?

No. During the Spires Fire step of the Turn the controlling player chooses a single Spire that has not made an Attack and has an Opposing Unit within its range, declares which Unit they wish to make the Attack against, and resolves their roll against that Unit. They repeat in this fashion until all Spires that can Attack have done so. This means if a Unit wasn't in range of a Spire at the start of the Spires Fire step but becomes in range later in the step due to a Talent or other game effect, that Unit can be attacked by Spires that have not yet attacked.