I think that you are saying that f there is one all-powerful God, that doesn't preclude the idea that there could be lesser Gods.
Yes, but here I want to clarify that the lesser would only be aspects of the Whole that have specific responsibilities - like Vayu (Wind God) , Agni (Fire God - 7 Agnis), Varun (water God) etc. whereas in the example given, Devi is Brahman but VishNu and Shiv are Brahman. They are the same one as energy-energetic, Shiv-Shakti.
If the strongest God is jealous, it might make sense that theists would not want to heed the lesser Gods for fear of retribution. They might even start to deny their existence.
You mentioned that Devi includes the lesser Gods.
Is it possible that a strong God could defeat the lesser Gods?
No. That would be very unlike Brahman, or a truly 'Almighty' God - who is not just a regional one but responsible for His entire creation. He would not use His might to fight His own sons / His own aspects and parts -- on the contrary He protects them.
The smaller god-pantheon approach VishNu (or they ask BramhA to approach VishNu for them) when they are facing difficulty from powerful evil forces. VishNu only intervenes to defeat the evil in that case.
VishNu nourishes, and protects.
I wonder if lesser Gods might move away, to a different part of the world, and become the dominant God of a smaller group of people? That might explain a tropical pacific island with a volcano God.
What appears as multiple gods within Brahman are One in purpose and spirit with the Almighty Brahman. They do not have conflicting motives.
The words dominant God, fighting a competing god -- are all foreign for Brahman. There is just One reality and all are on that reality like a fabric.
So the volcano god could most likely be assigned that place or gifted that place by the Almighty --
unless the volcano god is an evil force or a demon and not a god at all.
Now comes the question of different human cultures and their respective Almighties. There is only one Whole.
We cannot have multiple 100%s only one 100% can fit in a 100%
Hinduism understands , for instance, Shiva, VishNu and Devi as forms of the same 100% Whole. It does not mean there are 3 Wholes or 300%
They understand each other to be
Their Own Self.
Bring in the Almighties from other cultural views - they have to be the Whole, with perhaps a different form and name.
To be Brahman they have to be omniscient and know that
"God X is but another form of mine."
If they are any lesser than that - then they may feel the need to fight or
conquer each other like mortals! Jealousy etc. cannot touch Brahman by definition. It can touch celestial beings that are not Self-Realized.
Only time the devas fight is not for themselves but when demons or evil forces disrupt the balance of nature, and well-being of the world/universe is in peril.
They fight the demons, not inter-god fights.
Which is unthinkable in the Oneness of Brahman.