120 is a perfect multiple of 24, 30 and 60 so it can display all those content smoothly, plus I think ProMotion actually allows for adaptive refresh rate.
I believe the 60 Hz is fixed on non-ProMotion displays. 60 / 24 = 2.5 so it's possible you might notice some judder due to 3:2 pulldown.
All iPads pro (including the first) can go down to 24, but as far as I know, no non-pro iPad has variable refresh rate. Pro-motion is variable refresh rate that can go up to 120. Having said that most people won't even notice the difference for lower refresh rates, the main advantage is battery life...
Also, what's important for movie watching is subjective. For me 12in is the bare minimum for movies, so size is the n.1 factor for me (the 12.9 iPad can display slightly under 12in 16.9 movies, for 21.9 ones it's even less...)