Since cutting forces are inversely proportional to cutter diameter and speed for a given MRR/cutting power, a larger diameter and/or higher speed cutter might help ensure that your tests are router power limited rather than machine rigidity limited. This 1" diameter router bit at 30,000 RPM should enable 6 times more power draw (and MRR) than a 1/4" endmill at 20,000 RPM for the same cutting forces.
Although it’s designed for wood, it’s probably fine for easy cutting plastics (like UHMW) too. Using something like that (rather than wood) would make tests more repeatable and enable easier comparison with results from others, especially is a common manufacturer is used.
Here’s a couple of other thoughts on testing strategies from me and @wmoy.