Problem is the task is not just daunting it's impossible, you have chosen to rule, that they aren't lying when they say the Lower planes is infinite, which mean no amount of building will ever match them, we can build a huge realm, we can expand our realm until we have a billion billion subjects, and before infinity, our comparative might will still be functionally zero.
In normal D&D this is somewhat made up for, by the fact that this is true for the Upper Planes as well as the Lower, letting them keep each other in check, but here the infinite Upper Planes were somehow destroyed, we might be able to rebuild some of them, but we can't build an infinite Plane, no matter how strong we get we are a finite existence, so unless you conveniently rule, that there's a way to cause an infinite restoration effect, the only way for Evil to not be infinitely more powerful than Good, is to devastate Evil as much as Good is, no matter what the cost is.
