I'm afraid there's no way Glorious Anthem works at common. I've playtested custom sets where people put Glorious Anthem effects at common, and those cards just really skew things. This is a Glorious Anthem that ramps up to be Collective Blessing or larger? This would be perfectly suited to rare.

Note that Overload from Return to Ravnica gives us some examples like Mizzium Mortars and Cyclonic Rift where one mode of the spell is a common effect, the other is rare, and so the overall spell has to be rare. The same will apply here.

You may be interested in this discussion over on Goblin Artisans about the Illustrate mechanic, which is very similar to your Ambient mechanic. There was a lot of disagreement in the comments about whether it's possible at all to make commons with Illustrate, whether they have to be ridiculously low power level, and so on. That thread will give you lots of food for thought as you work through your designs here :)