Do you have a wattmeter? Those solve several problems in one go and will give you power in and power out, DC and AC.

You are expecting a maximum output of roughly 20W at 5V. I would use 2, maybe more heavy heating 10 Ohm resistors and do several combinations to get the maximal power. One testrun in parallel, one serial, one with just one resistor. Thick cables, proper soldering,... tedious work, but doable. And easily comparable results.

Resistors, even those made out of Konstantan, drift with temperature, sometimes as a function of the material, sometimes due to thermoelectric effects. Using just one multimeter, that drift can influence the result as the resistor heats up. A really big resistor with a large cooling area does not heat up that much from 3-5-20W and should be safe. Ridiculously big ones are available and cost less then a proper second multimeter.

With a wattmeter (or if you only do output and assume pure DC: 2 multimeter) a series of small 20W resistors should be enough. They do heat up, so put them somewhere safe. Or use variable, manual resistors. (I would pick a ~100W model, 2 multimeter minimum)

Electronic loads would be total overkill, but offer the most flexible setup. Pick one that can do MPP-tracking, it saves a lot of work.