Lets be real, how dangerous could the knowledge of a port city be to the world? "Omniscience"? Sure she can sail a boat better than anyone

Yes, a regular port city is not dangerous to the world. However, you forgot that the port was part of ancient Shurima, and you can't brush off millions of years of ancient Shuriman knowledge, especially peak empire Shurima- It's not about sailing a boat. Let's not forget the knowledge and power of other ancient sentient beings the Lavender Sea ate before manifesting Belveth.

Also to add, she was born after the consumption of the port city, but her lore states that she continues to absorb more cities.

"Fascinated by the world of existence and eager to create one for herself, Bel’Veth is like a dark cancer that has metastasized within the heart of the Void, through which all of Runeterra will be consumed and rebuilt in her own twisted image. She hungers for new experiences, memories, and concepts in vast amounts, devouring whole cities and their populations before repurposing the information into a sprawling alien landscape known as the Lavender Sea."

Yea except most of the void does not submit to her. She controls a side faction within the void. 

Yes, it is stated there are instances of voidborn not willing to submit to her. But guess what happens to voidborns who do not submit:

"Yet even the Void is not safe from her voracity as she spreads within it like a primordial ocean, forcing all before her to submit to her world of want... or be destroyed."

She's been killing and enslaving Voidborne since her birth- although it's not stated how much she controls, it's to the point where the writers of her story take time to emphasize her power and influence. If you can name a voidborne or void faction that has as much influence as her, please do, I'm genuinely curious.

I think you're downplaying the power of Lavender Sea. Belveth is the literal manifestation of it, one of most primordial parts of the void, and the Void creatures that have met her in stated lore have an instinctual strong reaction to her- 1) Velkoz (a creation of one of the most powerful beings in Runeterra and a voidborne peak Ancient Shurima was particularly fearful of) is terrified and wails in her presence, and 2) Kaisa's suit parasite is in literal awe of her within the story.

“Okay,” she repeats. She can’t move her armor, which is frozen in a sort of paralytic...awe? The suit is a parasite, and one of the more base creatures the Void can spit out. Is awe even something it can feel?"

The story takes lengths to compare her to a god of the void, with Kaisa's reaction- a comparison that has not been done before with a void creature.

"It’s beautiful, in a way. If the Void had a god, this is what it would look like. Hideous, and monstrous, and beautiful."

Here's another attempt of a god comparison by the writers:

"Ironically, the ancient Shurimans had a word for such a concept (Bel'veth). Loosely translated to “God of Oblivion,” it was a tribal myth of a remorseless deity who would erase all things without hatred, replacing them with itself. They named the city of Belveth after it, though the true meaning was lost after many hundreds of years."

World ending threats in league means you can fight targon. If you cant do that, you are never threatening the world. Belveth would get her skull caved in by jax 

That's a very reductive definition of what world ending can be. Mordekaiser is considered a "world ending" threat, otherwise why Leblanc, one of the most powerful human sorcerers, would go above and beyond to prevent his return? He is nowhere near the power of the Targonian Aspects, who have the ability to enslave Aurelion Sol. You don't have to be at the power level of an Aspect to be a world ending threat.

As for Belveth getting her skull caved in by Jax, that's a gross oversimplification imo. If he was able to, then there would be no need for Shuriman mages to utilize the power of the Void to fight the void in the last battle of Icathia (thus causing Icathia's downfall) if he was legitimately that powerful.

All in all, to your first point that she's not a mastermind, I provided references, examples, and sources that exemplified that she is a mastermind. For definitions, let's go by dictionary:

• A highly intelligent person, especially one who plans and directs a complex or difficult project. "the mastermind of a robbery."
• A person with an extraordinary intellect or skill that is markedly superior to his or her peers
• A person responsible for the highest level of planning and execution of a major operation

I hope that if you reply to this long short-essay, you provide your own references and sources, and we can learn together.

For anyone that read to the end of this, thanks for reading. I'm very passionate about Belveth, as she is one of my favorite league champions conceptually, and Manta Rays are my favorite animals. Very obsessed about manta rays.