I have a lot of gaps too, mostly in that I've never played an editions earlier than 3rd
The point of the material world vs the outer planes is a weird one. All of the Gods live in the Outer Planes, but they care about and interact with the material world. Same with devils and demons. Asmodeus lives in the Nine Hells, and most of his plots involve the mortal world.
So, both groups are associated with an outer plane, both groups are focused on and concerned with the Material, where they have worshippers and devotees.
And then we get to the sticky part. If the gods are seperate and don't require worhsip to be powerful, like the Nerull cults mentioned earlier, then they are like the Demon Lords and Arch Devils.
If the gods require worship to exist and grow more powerful from it, then why do Evil Gods exist and are equally powerful? Most of the evil gods are worshipped by small cults, just like the Demon Lords and Archdevils are. In fact, I'd put forth that Asmodeus has more cults and worshippers than Nerull or Umberlee. So why would they be gods and he is not?
And round and round we go, trying to justify a difference that I think is so murky by this point in the game, that it doesn't have a place anymore. Just make the Demon Lords and Archdevils "as unto Gods" mostly by weakening the good gods to not be all-smitingly-powerful, and continue with the Evil gods fading away.