This is about cutting forces as well - slow your feed rate, and there will be less runout and deflection. The combination of the two consumes cuttters faster than anything. You should be able to hold ~3-4 thousandths without much effort - as you have shown, the trick is to keep cutting forces down. I’ve noticed the steel belts also stretch quite a bit less when tensioning than the glass belts. The kevlar belts I find to be ok, but I’ve broken a couple, and haven’t broken any of the steel ones.
Also keep in mind that holding torque from the steppers (as well as stepping torque) is directly related to available current from the drivers. The stock drivers are adequate for the stock motors. If you want to go much bigger and upgraded driver will be needed to get all the toque the stepper is rated for (which is a lot of work to do - you need to go to an external driver, which really necessitates a different controller board) . It’ll still work, and you’ll still get more from an upgraded driver without it, but you’ll get more with a properly sized driver.