Seems to me you have a few mechanical issues going on.
The non-square corners may be cause by the gantry not being square to the frame. Check that the full length of the gantry hits the bottom of the frame at the same time.
The wobbling effect may be caused by the Z adjustment not being sufficiently rigid or if you have the laser module extended too far and acting like a pendulum oscillating back and forth on the gantry. Do a full mechanical review to eliminate any unwanted movement.
Can you close the gap between the wheels and the rail? There is likely an eccentric nut on the shaft of the wheels that allow you draw the wheels closer to the rail.
Is the laser module now well secured to the gantry?
Separately, did you check to see if the gantry was square to the frame?
But I suspect the fundamental problem is you’re simply attempting to burn too quickly. There’s no way you’ll be able to get anywhere close to 10,000 mm/min on a cutting operation, especially when you’ve just added a pretty heavy mass to the machine.
In the material test photo it’s apparent that you’re also missing steps somewhere. It’s possible this is a direct result of attempting to accelerate too quickly. If not due to acceleration being too high then likely something mechanical. I’d suggest revisiting belt tension.
If you’re getting different behavior at different parts of the machine then this is almost certainly something mechanical. I suggest checking carefully for differences in the setup between good and bad points. This is likely to be something related to Y-axis.
In any case you absolutely won’t be getting anywhere close to 10000 mm/min. I believe Ortur advertises the machine as being able to do 10000 mm/min maximum engraving speed. That claim on its face is plausible but even then quite fast. But note that this assumes a scanning operation. Simple left-right movements. It would never get this speed for more complex movements especially anything that involves the Y-axis.
You’ve now added a tremendous amount of mass to the machine. So immediately this cuts down the maximum speed of scanning operations let alone cutting operations.
Not very much. As long as all slack is removed and the belts taught they should be fine. Do not overtighten. The issue likely lies elsewhere at this point.
I can’t tell from just looking at the photo but it looks the wiggles are happening when quickly transitioning between straight cuts with the issue still in the up/down direction. You may for some reason still have a pendulum effect going on. Not sure why more at the back than the front though.
If you raise the Z height so that the laser module is more rigidly held do you still experience this issue?
