Great work well done !
For comparison my Y axis is sweet running like yours, and my dual screw X axis is much noisier. I think it’s because they are longer and there is much more going on.

The slipping in the second video sounds like classic stepper stalling to me. The stepper torque just falls away with speed so if the mechanicals are good then just drop the speed back.

Well done for getting this far it’s a great achievement !