the luminous outset became the interminable middle

That makes complete sense. I do remember part of A:TLA where Aang had to learn the method of learning each type of bending and there were different stances, but they also had Zuko adapting waterbending techniques with his fire (I think) when he was learning from Iroh. There's just so much fanon around the nature of bending that I wasn't sure if there were rules like Earth and Air don't mix, or exposure to other elements have to be present to afford a chance of gaining that ability. I do hope we see more blended families in this series.