Well with respect to Aang we have seen him go into avatar state without control as early as episode 2 when Zuko knocks him into water. And from then in the air temple when he sees the monks dead bodies , Roku taking over his body , the season finale with Aanzilla , when the earth bending general forced him , when Appa is kidnapped and so on. He also had a long one to one coaching from the guru .
We had an entire episode with him unlocking all his chakras . After all that he gained the ability to go into the avatar state at will in season 2 finale before Azula crisped him . Then he lost the ability to go into avatar state at all , then in season 3 finale somehow a rock pokes him and he regains the ability .
For Korra this was the first time she made contact with Aang. The equivalent ATLA equivalent is the first time Aang made contact with Roku in the spirit world when some spirit bear was ravaging and kidnapping people . And immediately she gains full control of the avatar state. No foreshadowings , no involuntary entering into the avatar state , no unlocking chakras . It was rushed up. But as you said it is understandable as it was supposed to be a one season stand alone show.