Safely measuring voltage from solar panels in an experimental research field
This is my main plan so far, which I'd like feedback on:

Wire all 9 panels in series to minimize amperage


Putting the panels in series will boost the nominal voltage to 200 volts. Frankly, I'd recommend 3 sets of 3 panels, rather than 1 of 9. This way the nominal voltage will be about 55 volts.


Use 14 AWG wire to take the power out of the field to some sort of resistive load to use the power


14 ga wire has a nominal resistance of 2.5 mOhm per foot. This will give a nominal voltage drop at 5.38 amps of 27 mV/foot (over both wires). You don't say how long a separation you want between the panels and the load.


The unit will be grounded using grounding rods


Why?


On the circuit, there will be a 10A circuit breaker, a manual on/off switch


OK, but keep in mind that you'll need to specify DC circuit breakers. AC circuit breakers count on the fact that the voltage across the contacts falls to zero 100 or 120 times/second, and this reduces the arc length required for quenching once the breaker trips.


The resistive load will be connected to a GFCI


GFCIs don't work on DC. Plus, it's not clear why you would need one in this application.


Power will be measured using some sort of Arduino based module


How do you propose to power the measurement box? Will you run an AC line to it? Will you attempt to power it from the solar cells? Will you provide it with its own solar power system?
As for your load, I'd just go with some big-ass power resistors. If you use 3 circuits of 3 panels in series, your nominal load for maximum power will be 10.37 ohms at 300 watts. I'd be inclined to get a total of 15 ea, 50 ohms at 100 watts, such as these https://www.vishay.com/docs/31838/fvt.pdf They are in stock at Digikey, and would cost a total of about $180. Mounting is simple, and you could make a rack of them with a small roof overhead to keep the rain off. For each circuit you'd put 5 in parallel, and you'd have a 500-watt, 10 ohm resistor, so you'd have plenty of margin in terms of power dissipation. You'd expect something like 1 - 2% resistance variation over temperature, so you wouldn't need current meters. Just measure the voltage across the load.
Look, for measuring solar power, don't try for super-high precision. A few percent is all that makes sense. You can't control the amount of cloud cover, and it will vary from month to month and year to year. So don't overthink your data requirements. Keep it simple.