Mini LED is inferior to OLED so yeah you'll find the 12.9" screen will be worse than your 12 Pro Max.
OLED - can turn off individual pixels for infinite contrast.
Mini LED - uses Local Array Dimming. the only difference is that there's a lot more LED backlights but these LEDs are still waaaaay to big to light up individual pixels. this causes "blooming".
say you are displaying this image on your iPad:
an OLED would be able to turn off all the pixels that are black and light up only the pixels showing stars. a Mini OLED would divide the image into thousands of small sections and light up the
sections and not pixels. where the stars are the LED would light up but what you'll see (unless the star is just the right size) is that some of the black pixels surrounding stars will also light up too! this is the blooming effect. and not just that... if you're watching a video the LED controller needs to be really good to keep up with the video footage. if it's too slow then you will also notice a lag. say you're watching a video of stars in space then if the LEDs are too slow as the the star moves then the areas where the star was will remain lit up for a while.
videos showing this effect (skip to 0:53 if it doesn't start there) :
the screen on the left is using Local Array Dimming on an LED display and the TV on the right is an OLED. look at the blooming around the circle on the LED. while a mini LED will have more smaller LEDs it's basically the same technology and that's similar to what you'll see on your 12.9" iPad Pro. Apple should've went OLED.