@NotFastEnuf: Agree 100 percent on 8.5mm brushed being too heavy for (normal residential) indoor flight. Success of the Tiny Whoop and most activity here on MMC shows clearly that cutting weight to the max offers most satisfying results.
Interesting question! I never had an FPV version of the NQX, so don’t know how the 6mm motors handled the extra weight. My suspicion is that with a heavy AUW, 6mm motors would not have enough torque to spin up larger props fast enough to fly aggressively. So, as you’ve already realized and applied, cutting weight to the max is what will make or break this project.
My hope is that with enough refinement of the frame design and powertrain, you will succeed. Worst case, you may have to abandon 6mm and use 0716 motors from the E011 for more torque and efficiency.