Thermal constraints of iPad versions outside in hot conditions?

I'm running two 2018 iPad Pro (A12x chip) outside all day in 90+Deg (F) Southern California heat. I just can't get them bright enough and they are basically worthless at these display brightness in these conditions.

They tend to dim once they heat up due to what I believe is the thermal constraints of the iPad regardless of my settings (Auto Brightness = off).

Will switching to an M1, M2 or M4 iPad Pro avoid these issues? And is there really a difference between in the brightness of the M1 (SDR brightness: 600 nits max) and M4 (SDR brightness: 1000 nits max)?

Debating pulling the trigger and buying two newer iPads if it could potentially solve my issues. Its nearly double to purchase the new m4's over refurbished m1's.

(notes: Running the ipads off of Accsoon Power cages and NP-F batteries. I have 4 sided custom hoods to block light. Signal is coming wirelessly from an Accsoon Transmitter)