I totally agree with your analysis of the ATLA finale! It has a real feeling of conclusion that's just so heartwarming and satisfying.
I sort of agree that LOK should've ended with the Book 2 plot, but I feel the plot might have to be tweaked in order to make it more "series finale" material. While Book 4's finale didn't have a concluding feeling either, I think Book 2 felt like a transition finale, what with the Spirits being welcomed back. TBH, the only LOK finale that really had a feeling of "conclusion" to me was Book 1. There were obviously more stories to tell, but unlike the other books, if the story had been dropped at that point, I would have been satisfied, because all of that season was tied together in the end. The others didn't have that feeling. So, if Book 2 was the end, I think it would need to end on s more conclusive note, without loose ends.