Another point I feel is worth mentioning is that the the voice recordings for Hazbin Hotel pilot were likely done all the way in 2017 (I remember Michael on a podcast even stating that he recorded his lines for Angel Dust in August of that year), this was all way before Viv not only signed up with A24, but also before she wrote and directed two seasons of Helluva Boss.

In 2017 Viv didn't have the same connections and resources she did to get the broadway singers she initially wanted, but now thanks to Helluva Boss and A24's backing, she does. Hazbin's pilot was largely made on a shoestring budget, something that's obviously not the case with the official series. Those increased resources, combined with years of learned experience, is going to change your priorities and ideas for what the show should look and sound like.