So you got it working perfectly, impressed the heck out of all of us, then tried to double the depth? All at similar speeds and step over? Killing me smalls.
You are allowed to take multiple passes.
I think the previous video is seriously pushing the maximum for a machine the size of yours. You had amazing results, stick with them and see how it goes. Don’t forget the deeper it goes the less rigid it is on the MPCNC, the lowrider is opposite.
A few things to consider. I would try the single flute again. The more flutes you have in a deep cut the less time you have for a chip to get out of the way of the next blade. A deeper cut also should be run a little slower for the same reason. Air could help but slowing down the feedrate or the spindle rpm will help as well.
Results at 2mm will not be the same as results at 8mm, CAM is a tricky tricky thing.