How to control 4 thermistors in a heated bed? My print area is ~ 17x16 inches (431.8 mm x 406.4 mm = 175,158.4 mm/2). To that end I need four (4) 200mm square heaters, and associated mosfets to run it. Each heater has it's own thermistor and can potentially reach the target temp independently (theoretically they could, since there will be deviations in manufacturing).

My mainboard (the SKR Pro) has support for 4 thermistors; however my solution in this configuration has 5.

When I double the size of the printer later this year, I expect that I will need 8 heaters.

Is there a solution to control that many heaters such that non of them over shoot their temps, or am I going to have to design and build a dedicated thermal management board to control beds of this nature?
You can use the Klipper firmware which accepts an arbitrary number of heaters.

Then you can write a Klipper macro which takes one input and applies it to each heated bed (or not, as you prefer).

With Klipper you can also add another Arduino and connect it to klipper as well, in case you don't have enough pins on your board: klipper is not limited to controlling one single board per printer.

Check the docs.