1-3) Google “chip load. Don’t use the numbers that come up, but it will explain why faster rotation, and slower cutting isn’t always better. You bit “popping” out shows you have a big issue. If you are burning the wood you are going too slow, or spinning too fast, or both. Is your tool spinning the correct direction? The speed control is a great add on. I believe in that oak ply 6mm I was cutting 15-20mm/s 80%rotation speed. For MDF 6mm I actually was at 15mm/s 100%rotation.
4) The belt doesn’t stretch under normal load, it has a 6.25lb working tension and a 120lb breaking strength. The zip ties tend to bend into place pretty quick and have a 40lb rating. so tensioned right the zipties should not flex neither should the belt.
So the big giant red flag I am getting from all this as counter intuitive as it seems is your assembly. So tight you heard it crack? Some of those bolts control tension to the rails. Is it so tight that you are causing extreme drag on your rails and the steppers are flexing them? The machine I built last week, I actually hand tightened all the long assembly bolts, no tools. This machine is plastic, it is designed to be very loose. You first video seemed like it wasn’t fitting together right so I understand going the opposite way and over tightening things but the middle ground is what you want. Take your time. Do all the rollers move with the same minimal drag on the rails? How does the center assembly feel without the rollers. Do they all still feel smooth when fully assembled without the belts on? And the parts you added didn’t help so I recommend taking them off, except my mount, that should have helped so for a rigid mount making it worse makes me think you have a z axis problem. Are your z rails parallel? They can get twisted (kinda spiral) and cause crappy cuts. If you take the z axis off and lay it on a flat surface do your rails lay nicely or does it rock?
This isn’t a common issue. Most builds work very well from the get go so I am looking for the one little thing you must be doing differently.