The Dark Dimension is not just a single dimension; it is a collection of realms all conquered by Dormammu. Each of those realms may have a distinct geography, population, etc.

The Dark Dimension is an amalgamation of a bunch of dimensions and realms. It's not supposed to be consistent, if that makes sense.

Edit: Jinx, my bad.

Those shows didn't have the budget necessary to have it appear as it does in Doctor Strange.

In the "main" dimension also called the Earthly plane there are many different looking places for example New York and a random place in space

I mean AoS full on did the Quantum Realm. They absolutely have the budget. They just never went that deep into the Dark Dimension.

I guess because what we saw in Doctor Strange, Agents of S.H.I.E.L.D. and Runways are different sections of the Dark Dimension.

With the Dark Dimension pretty vast, the three shows showed us three different parts of the dimension

Perhaps som since it seems Dormamu ever appeared.

Actually, I have my own theory but it's kind of stretched

As all of the above mentioned, Dark Dimension is an amalgamation of plenty of realms and pocket dimensions.

But I've always wondered what was those "planetoids", as you called them, the ball-like things that we see in Doctor Strange.

And then I wondered : what if those planetoids were, in fact, the different realms, like if you could go inside them, like in a bubble, you would find yourself in a pocket dimensions, a realm of the Dark Dimension ? Like, inside of one of those planetoids is the Loa realm, inside an other one is the realm where Morgan Le Fay was exiled, etc, and what we saw in Doctor Strange was just the entirety of the Dark Dimension.

I think that's my headcanon about the Dark Dimension's design

That’s perfectly possible.

This will also probably be further explained in Doctor Strange 3 since Clea now exists in the MCU.