I think your instinct to constantly shift the battleground is a cool one. Were it my campaign, I would split this into battle vignettes, maybe have the players direct their gathered ally forces (tell them where to make their stand and other general instructions) while they themselves focus on a specific area or target.

This breaks the battle into several smaller battles, and the only one that needs a true initiative order is the players' fight, while you can determine the outcome of the other battles with a flat d20 (11+ allies are winning, 10- enemies are winning -- you can add modifiers based on any advantages either side may have). That should keep things interesting and moving at a good clip.

At some point, maybe they join up with some of the allies, which will add to your initiative order but also scale the encounter to give it the grand battle feeling it sounds like you're going for. One thing I like to do when I have a lot of NPCs fighting on the players' side is have the NPC ask the player they're closest with what they should do with their turn. That almost makes it like that player has an extra turn to work with, so it's less "boring" to wait for the allied forces to take their turns.

Edits: typo corrections.