If you take the dewalt mount off you should be able to get a square in there pretty easily.
Or you can just figure out your cam. I feel like you want it to be something else. You can get things progressively more accurate but with your previous video I am telling you your machine is spectacular. Just work on your CAM program.
You are doing this the hard way and going hardcore into 3/8″ aluminum, most of us have spent 99% of our time in wood, then some plastic and a bit of aluminum or brass, you are insisting on pushing the limits in the beginning. There are so many subtleties to learn. You seem to be frustrating yourself. Maybe you can step back and make some things out of wood, or even test your gcode in wood, then acrylic, then metal.
The MPCNC is going to take more precise CAM than a machine that has a few more zeros in the price tag, but honestly, not by much. Add a few more zeros still and then yes aluminum gets a bit easier. If we could do steel you would see it get exponentially harder still. Any time I have been around large machines, the program gets checked by another programmer before they try and run it if it is steel.
I have not run the same program twice yet. I run it, watch, take notes (seriously), tweak the CAM, Run the next job. So many sets of lowrider flat parts and I am still changing it every single time. And that is .25″ wood.