In my opinion, how can Necropolis be better balanced?

instead of having monsters from levels 1-4, have monsters from levels 4-5, but in smaller numbers, so that the difficulty of defeating them doesn't change.

I completely disagree! The difficulty of defeating them increases many times over because the first three levels are a joke! (except for a few Warriors). And the number of skeletons will decrease, and you'll end up losing them all while trying to defeat all these endless guards.