The display is graphical, and has to respond to the slave states, the keypad, and the master state.

Sorry, that should have read 'the state of the master'.

If the current master processor needs to make a lot of filtering and decisions of the key presses and measurements before being able to decide what should be drawn, then there can be a reason to separate that functionality from the functionality of actually performing the draw primitives on the display

Yes, that is the case

Divide and conquer works best if both sides of the split has a significant size.
So the question then is if there will be a significant amount of work/code/state storage left for the original master to handle, if the display management is moved to a new processor.

Yes, the master has a lot to do without the display. There are > 10 inputs to scan, and decisions made on the input conditions. Then also the RS485 network.
Some of the inputs are rotary encoders, which go to interrupts. For instance, on forward movement, weedkiller has to be sprayed, except where planting is to be done.
The master is busy.
The display is a 128 x 64 Graphics LCD with ST7565 controller.
Fonts must be looked up, etc.
Thank you
Johan Smit