Avatar was very impactful, but you are not incorrect that the first season is generally inconsistent with a lot of tonal shifts. Avatar definitely succeeds the most in Book 2 and Book 3 where it feels the characters are growing up with the audience (and I'm your age Fab so I'm not sure that argument sticks its landing), which is a strength of the material. While watching one season is more than enough content, I do think its doing the show a disservice if you don't at least spend more time as the arcs get more mature and at times darker and more nuanced. It definitely does pacing a lot better than Korra or Dragon Prince and it does reward adult viewers well enough.

I think a large part that doesn't help with Avatar is that its reputation is rather inflated due to its influence and significance for [western] animation of that period of time.