I will take the risky and frisky action and (slightly) disagree with the Master.
I used a real-world exercise when choosing our current chart plotter. I was allowed to borrow several new plotters from the local marine electronics store, and (handing them with paranoid and utmost care...) trial fit/position each one above our compass under the ss guard. (I did have the guard extended on both legs to give more room for the frp shelf and 9" plotter.)
FWIW most of these devices have a "compass safe" distance specified deep in their PDF install instructions. A real marine electronics shop sales person will discuss it with you, and a typical sales droid at a big box marine store will not - and often knows not.
Several install manuals I have viewed for differing brands mentions 19 inches as a safe distance. Closer might be good enough, and their estimate is likely a compromise between their engineers and their liability lawyers.
Only the one model with a friction-catch chip door did not visibly "move the needle". It's a Lowrance and was from the same parent company that produces several very similar brands for differing boating "markets", and the shop said that the electronics were very similar.
I could have chosen a Simrad or B&G unit, based on similar features. However, those had
powerful little magnets holding their chip door closed.
If having to fall back to old-fashioned analog navigation when power fails, one could always just unplug and unmount the plotter and store it below decks, I acknowledge. Nothing wrong with any of your choices, but be sure you understand the ramifications and
limitations. (Insert Clint Eastwood movie quote at this point!)
Edit: the internal circuitry itself -when powered up -may also cause a magnetic compass error at some close distance, and that's really a separate concern from the gross effect of the permanent magnet problem. Something else to discuss with a technician.