Why did Avatar: The Last Airbender end the way it did?

Ok so recently I watched a few episodes of Avatar and I was just got instantly hooked. With school, friends, and responsibilities I managed to finish all 61 episodes in less than 2 weeks. I'm not going to go into too much detail about how I feel about the 4 episodes of Sozin's comet but here they are:

1) Why was it so sudden and rushed?

Avatar was building up the series in a very coherent way with a steady pace and all, but when it came down to it, Sozin's comet was a rushed 4 episodes I think. It went from Aang spending a better part of 3 seasons/books trying to master all 4 elements to one day fighting the formidable Fire-Lord Ozai who was "The baddest man on the planet" to Aang actually fighting the Fire-Lord without even having mastered them all yet. In the show Zuko and Toph both agreed that Aang needed more help in firebending and earthbending which makes it seem like the fight between them that much more premature.

2) Aang vs Fire-Lord Ozai

When the big moment finally comes where Aang actually fights the Fire-Lord, why does the fight last such little time. Not only that but the fight is very unsatisfying, which I believe many people would agree. I think that having 2 events of Aang vs The Fire-Lord AND Toph, Suki, and Sokka fighting the airships was a bad choice because it doesn't give the fight the "Holy crap, this is it!" moment that the show deserved at the end. Aang taking Ozai's bending away was a good choice I believe though because it fits Aang's character but there is no epic moments during the fight that were satisfying.

3) Why are so many of the Characters Neglected in the final episodes?

During the last few episodes, many of the characters are not given enough attention to. For example, Iroh is one character who played a major role in the first 2 books, but had very little to do with the 3rd book. I would have loved to see more of his understanding and wisdom on what was going on during it.

4) Relationship Outcomes

I understand that the main audience of the show are kids but they should at least try to appeal to the more mature fan base. It was very forced in my opinion that Katara went with Aang after only 5 episodes back during the one where they go see a play, she tells Aang that she is confused when he asks her if she thinks of him as a little brother or has actual feelings for him. In all honestly Aang is a little kid and I don't really understand why they put the 2 together. Zuko was another possible choice for Katara, I mean he is older than Aang and there were a lot of Zutara moments in the show. Could this have possibly worked out? I think that it could have.

5) Loose Ends

There were many, many things in the show that were up in the air when the final episodes came and left. For starters the writers should have either answered or ignored Ursa and her disappearance in the last episode because they brought it back up but decided not to answer it even though they knew that they would not make a book 4 and it was only planned for 3 books. Another one is Jet. They never actually tell you what happened to him (I assume that he died though).

6) Higher Level of Depth

It really awes me how the writers could end the story in such a simple and straight forward way. I mean I can literally sum up the ending in a few sentences:

Aang fights Fire-Lord

Aang beats and takes Fire-Lord's bending away

Toph, Suki, and Sokka beat airship fleet

Zuko and Katara beat Azula

Zuko becomes Fire-Lord

The End

Don't get me wrong I LOVE the show and I'm so sad to see it gone but I can't help but feel disappointed that it was not given a satisfying end with a good final battle, all loose ends tied up, and a higher level of depth so it wasn't so predictable and bland.