As for archdevils, it's established that Asmodeus is the face of the Nine Hells. Even powerful underlings like Mephistopheles and Glasya have a hard time forming cults since they all answer to Asmodeus.
One thing I did for this, was to make the Nine Hells much less backstabby, and have each circle and ruler focus on different things. There still is backstabbing and betrayal (they all know that if they can defeat Asmodeus they get his power) but I just tone it down
So, Glasya's cult focuses... Malbolge, so I made them the "drugs and medicine" aspect of the hells. They deal with "cures" and peddling addictive substances, which gets a very different type of follower than Mephistopheles who focuses on arcane research and knowledge.
Gods often have some nebulously-defined ability to affect the entirety of reality just by existing.
The presumed ending of the 4E Scales of War adventure path depicts the ultimate destruction of Tiamat as lessening the capacity for greed in all mortals. A specific example given is that even the temple of Bahamut becomes more charitable thanks to Tiamat's true death, going so far as to get rid of fancy religious iconography. However, it also states that another entity, such as the archdevil Mammon, may attempt to claim Tiamat's former power over greed, rise to become the new god of greed, and through godhood inspire greed in the hearts of mortals again.
That's why Orcus and other lesser evil entities desire godhood. Why would he settle for animating the undead personally when he could kill the god of death, claim dominion over death, and then change the rules of the multiverse so that every mortal who ever dies reanimates as a ghoul or something?
There are massive problems with this idea though.
Take Orcus and let us say he kills Nerull. Nerull was an evil god of the dead who wanted to destroy all life and created undead servants to kill people... So, if Orcus becomes an evil god of the dead and wants to destroy all life by creating undead servants to kill people...
Doesn't that raise the question of why the heck Nerull didn't do the same thing? What is the point of Nerull and Orcus existing in the same setting when they are doing the same plots?
Or, take the idea that Bane is the source of all Tyranny. That means that if Bane is destroyed, the Nine Hells become less Tryannical? Asmodeus becomes less Tryannical because his entire source of power is tied to a God who is above him?
It highlights this issue in really weird ways when you essentially have these massive end-game bosses who are threats to all reality... but the concepts of the game imply that they are actually reliant of the power of entities that stand in opposition to them?
Why have good gods when there are angels (solars, planetars, devas)? Or lawful gods when there are modrons? Or chaotic gods when there are slaad?
Aside from a difference in power level and origin stories, I think you are getting hung up on the "tangible interfering" bit
Snipping most of this to focus in on your first bit.
I can name about 16 unique and powerful threats between the Archdevils and Demon Princes. Each of them ruling their own planes of existence with massive cults of worshipers for them per the rules of the game.
How many Angels rule their own Divine Realms? How many unique and powerful Slaadi can you even name?
This is the problem to an extent. If Orcus is only as powerful as a Solar, then why does he seem like this massive threat to the multiverse? He's just a servant of a god. How did he even create new forms of undead and end up ruling a layer of the Abyss? why isn't Nerull in charge and smacking Orcus around like a schoolyard bully?
Or Asmodeus. As one poster pointed out, he owns and runs and entire section of the Great Wheel by himself. The Nine Hells of Baator all bow to his will, but he is supposed to be massively categorically weaker than the dozens of gods that share Mount Celestia?
That doesn't seem to make sense.
Not everything has to have "a use". Some things just exit. See: quarks.
And evil gods do not all have "evil cultist" followers. Some have hard-working priests busy interceding to stop the evil god stomping on people. Evil is not a synonym for antagonist. the world is full of evil, but most of it isn't trying to kill you.
1) When designing a world, having things exist "just because" is poor design. It needs to have a reason, or why waste the space creating it
2) Sure, Umberlee is "evil" because of the cruel sea. But, why not just make her neutral and moody? After all, the ocean is the bounty of life.
See, she is so specifically evil (the cruel ocean in a storm that destroys things) that you can say she isn't an atangonist, but in that case why not make her the neutral goddess of the seas? Sometimes she is beautiful, calm, and grants life and boons to the land. Other times she is wrathful and destroying everything. That actually fits better into how an ocean diety would be worshipped.
But, now take Zehir. Evil god of poison, darkness and assassins...
Who shares Darkness with 10 other entities, at a minimum, poison with 2 others and Assassins with at least 2 others. Most of them other gods.
And there are demon lords and archdevils of darkness, disease, poison, snakes, assassins, ect ect ect. Why so much overlap?
Because the sea is cruel. People go out to sea and don't come back. In polytheistic religions people try to find meaning in natural misfortune by blaming cruel gods. And they try to avert misfortune by appeasing the god with sacrifices.
If the god of the sea was good no one would ever drown.
And those alignments are symbolic of what they represent: the Sun is good, storms are chaotic, judges are lawful, etc.
If the god of the sea was evil, all fish would be poisonous and no one sailing upon the water would come back alive.
And why is the sun good? The sun can be a terrible force of destruction.
This is actually why I removed all nature deities from my game world. Most of the them became powerful spirits for the "old religion" of the druids. Because nature can be neither good nor evil, it is neutral and having a single god rule over that aspect just seems silly to me.
There is no reason why a tropical island dwelling culture in the Forgotten Realms couldn't worship a benevolent god of the sea. And FR does not say to anyone "your god aint real", so you could imagine different gods of the sea clashing as their worshipers clashed.
Which differs from Theros. Theros only has one god of the sea. If someone worships a different god of the sea, then that god isn't real.
This doesn't really work though with the idea that the dieties are their portfolios. If killing Umberlee removed seas from reality, because she is the sea, then how do you justify three, four half a dozen sea gods? Are they all the sea?
And if they aren't the sea, and are competing... why am I worshipping Umberlee to leave me alone when I could worship Valkur, one of her enemies, and ask him to protect me from her?
And, if I now have a god and the destructive enemy of that god no one is worshipping... Why do we have Umberlee instead of Demon Lords like Dagon or any of the dozens of others involved in being the lords of evil underwater creatures.