In GoH, we kept the differences but gave the Lords of the Nine increased cosmic rank (effectively divine rank) while on their own layer. That allowed the politics to still play out.
I think you could make them all Intermediate Deities - and that might actually make more sense in the context of:
1. The Hells vs. a Pantheon etc.
2. The power of an Archduke vs. The power of a Pit Fiend (CR 20).
That said, my 5e Pit Fiends are CR 12, 16 and 20. CR 16 would be Knights of Hell, CR 20 would be Barons of Hell (slight Doom game reference there). With the Companies of Pit Fiends formed from the CR 12 version, not the CR 20 versions.
If Asmodeus is a CR 28 Lesser Power, he is roughly equivalent to three CR 20 Pit Fiends* which seems a bit rubbish.
*That's three Pit Fiends rebalanced with my rules - not the flimsier Wotc versions of either 5e or 5.5.
However, one issue with boosting them to Intermediate power is that you have to do the same with beings like Orcus and Demogorgon. Again this may well be preferable - in fact the more I consider it the more I like the idea.
However, Lesser Gods (CR 28) are still applicable foes for semi-mortal Godlings (Hero and Quasi-deities) to have a go at. Intermediate Gods (CR 32) probably require full Demigods to take them on.
Typically I'd suggest:
Elite enemies = same rank
Mini-boss = +1 Rank
Boss = +2 Ranks
Solo Boss = +3 Ranks
So if Orcus is Rank 4 you want PCs to be
Rank 1 (to fight him on his own).
Rank 2 ro fight Orcus with some Elite Bodyguards (Lich, Balor, Molydeus, Nightwalker) etc.
The number of Bodyguards equal to the number of PCs minus 1 (or 2 if you want to be generous).
I agree that you can't simply have differences in divine rank and expect the more powerful Lords not to simply take advantage.
Numerically the Demon's should have more deities of each Rank. But the devil's are more organised.
666 Demon Lords
66 Demon Princes
6 Demon Monarchs
99 Dukes of Hell
9 Archdukes
88 Daemon Masters
8 The Diseased Eight
It's possible Asmodeus is the only Intermediate Rank Devil (Lucifer being a Greater God).
Anthraxus/Poxpanus/Famine might be the only Intermediate Daemon in Hades (alongside Hextor/Eris/War and Incabulos/Oizys/Famine), with Nerull/Thanatos/Death a Greater God leading the Horsemen.
No right way to break all this down, but ways maybe make more sense than others.