Since my last post, I got some recommendations from @designtheeverything about how to improve the milling on the @pocket_nc.
He suggested something kind of counter-intuitive to what I was initially thinking but made sense if you thought about it. That was to increase the speed, keeping the sideload the same and depth of cut the same, on the face of it, if it doesn't work when the machine is running at 18 inches a minute, how would increase the machine load help? Well, the answer goes back to the main issue I was having, namely heat since I had it running slower, it gave the endmill a more extended period to cut those chips. This means that the chips were heating up, and before they could get cut, they transferred some of their heat to the rest of the stock, and little by little, this heats the aluminum to a point where it starts gumming the metal.
So I decided to do just what he suggested, and bumped the speed from 18 inches a minute to 40, immediately I had to stop the machine due to it stalling, the torque on the Pocket NC wasn't enough to keep up with speed at the sheer amount of material coming at that pace. Thankfully this was an easy fix, and that was to reduce the stepdown from 100% of the tool dia to 30%; this worked beautifully, the sound was pretty decent, a little rougher than I would usually like but it was cutting no problem. I then increased the speed to its max (60 inches a minute), and it improved the sound.