Plastic Nuts like the Dumpster CNC nuts are really only good for up to about 25lbs of force. At higher loads, they will have backlash, whether they are worn out or not?
Are you using a lubricant on them? If you oil them, then they shouldn't be wearing out that fast.
I certainly wouldn't be buying ballscrews from McMaster-Carr. You can get Chinese ballscrews for about 1/5 the price, if not less.
https://www.ebay.com/itm/brand-new-S...kAAOSwv0tU9So4
Rack and pinion might be a better choice, but would probably require a lot of mods to your machine.