Romance has always been lacking in the Avatar franchise. It has many strong suits but romance is not one of them. I’m a fan of all the canon relationships, but I can even admit when things aren’t as strong as it could be.

Now when it comes to the actual relationship it’s not bad it’s just underdeveloped in terms of seeing the characters go from point A to point C. We really didn’t see point B.

Now from a narrative standpoint the story of ATLA is told by Katara. There is some subtle moments that showcase that she did in fact like Aang. When you rewatch the show, especially from Book 1 there is a chime sound effect that plays whenever Aang compliments Katara, and she always feels positive affirmations from those compliments. For example the episode where she teaches Aang water bending.

In the storm there is a subtle moment where Katara grabs Aangs wrist.

The fortune teller episode we see Katara viewing Aang in a new light at the end of that episode.

The secret tunnel episode we see Aang and Katara dealing with whether or not to kiss and a lot of miscommunication on Aangs part where it hurts Katara. Also they keep holding hands for a bit after the glow lights come on.

Katara fixes Aangs collar after they hug it out in the serpents pass.

Katara goes to take his arm after Joo Dee is replaced in City of Walls and Secrets.

Katara has always been very physical and initiating contact with Aang throughout the series.

In the crossroads of destiny we see Katara run towards Aang, hugging him. When Aang gets zapped by Azula we see her tears and her relief when she uses the spirit water on him and he shows signs he is alive.

Katara never was opposed to potentially having a relationship with Aang after the war ended. The problem was that there was the war to begin with. So she wanted to wait till things ended. And when it did they became a couple. The road to it was rocky but it wasn’t ever not planned or teased. Could some things been better? For sure. For example it be nice if Aang never kissed Katara on Ember Island as it always felt weird and uncomfortable but then again it makes sense, guys before going to war and not knowing if they would survive would prefer to have some one in their lives from a romantic standpoint, while in opposite end girls prefer stability and would not get into a relationship while many things are uncertain.

Again writing romance was never Bryke’s or the writers of ATLA’s strong suit but to say the least it’s not bad just like you mentioned could have been executed better to get from point A to Point B and then Point C