Thanks,
I had no idea ebony was available at that sort of price, I assumed you needed to be Pablo Escobar to afford any reasonable amount.
As for belt tension, there’s an updated table in this post
The simple summary is that I did not find any reason to believe that there was any reduction in backlash or vibration with higher belt tensions once the minimum working tension is achieved. I was not able to get the rating data for the Shapeoko steppers but pretty much all of the steppers in this size range have a load limit which is pretty close to the minimum tension, after that the shafts will eventually work harden and fracture.
The minimum (and maximum) tension is set by the stepper max torque, both sides of the belt need to stay in tension, that’s about 18lbs which comes out around 80N.
HTH