How could all deities exist at the same time?

[HopefulChild]

There is no dilemma. There is incongruity, based entirely on your own confirmation bias.

Why so many gods for the sun when one will do?

Sun gods don't drive/maintain the sun. That's silly.
The sun is a star, and stars exist as super massive fusion reactors perpetuating a plasma engine based on gravitational collapse.

Ra is an Aspect of human existence, not the Sun itself, or the keeper of the sun. Masculine. Powerful. Life giving and Life taking. A personification.

The Goddess is not the Moon. That's silly.
The moon is a tidally locked remnant of a smaller planet or transiting body that crashed in to earth and became trapped in it's gravity well and created a binary axial dependency.

The Goddess is an Aspect of human existence. Mysterious and perpetually changing through phases that nurture life as we know it. The lunar calendar is more accurate time, than Sol/planetary bias calendar and it used to tell people when to plant, when to harvest, and when conditions would be harsh to life or helpful to life. That gives it a more Feminine slant (in classical standards).

If all these gods are allegorical or simply personifications, why is there belief in them?

Have you ever met a Luxembourgian? Luxembourg exists. You could go there. People live there, and they are called luxembourgians. Why do you believe in them? Because you can see it on a map? Because someone smart told you about Luxembourg?

If you don't want to believe in luxembourgians what happens to them? Do they cease to exist? Do they stop ubering, and texting, and making love, and killing each other for money or drugs in a back alley, or stop giving birth, and writing poems, and singing songs?

If you don't want to believe in gods do you think they fall apart?

It is fantastic that you have explored the knowledge of humanity and discovered that Atheism is lacking a component. You obviously feel pulled and conflicted because there is a voice inside you telling you that there is more.
Congratulations. Many people ignore or suppress that voice and never deepen their soul. I'd say keep thinking about it.
Then think about WHY you think some of the things you think...then tell someone about it and find out what they think.

[Shekinah]

True many fantastical supposed Divinities many derived in ancient history when humans were not very bright. We are still not very bright and believe in many things that could not possibly be true (biblical lore for one). But we cannot preclude the real existence of some other worldly entities from where ever we do not know. Extra dimensional, inter galactic etc. One day, if we survive ourselves, we might appear as a Divinity to primitive inhabitants on some distant backward planet.

[SnowCat]

My deities have always represented themselves as distinct beings, while still being members of a greater whole. My pantheon has shifted over the years, as dictated by life, but I have never sensed any animosity between the deities. And I have wworked with members of different pantheon side by side, without difficulty.

Snow