the final climactic battle is set against a backdrop of a kaiju fight between Liberty Prime and a giant mechanized The Motherland Calls

How about instead it's Liberty Prime versus like ten salvaged T-34s, which wreck it from outside its effective range because despite being extremely old and outdated tanks they still have actual tank guns. That would drive home the point that Liberty Prime was actually overengineered rubbish that served no actual battlefield role and was basically just a terror weapon to unleash on civilians or unsupported infantry, and which couldn't stand up to any sort of combined arms tactics.

This sounds incredibly similar to the S.T.A.L.K.E.R. series minus the external political factions.