Carpet will hold spilled powder, and if the pile is too high will conceal dropped primers (live or used).
Vacuum of very flammable materials needs something that won't expose it to the electric motor. I have a throw carpet I can remove from my concrete floor. When I do vacuum, I use a Dyson which isolates the air flow from motor. Even so, you don't want to be vacuuming a mix of live primers and spilled powder.
I think 3/4 plywood is too thin for the mounting area. Try using a 2x8 horizontal across the front edge of the table to provide a mounting base under the plywood, as well as more rigid mounting. Make sure that things cannot slip on the filing cabinet base you use...
I use a lab bench chair with a foot rest that I can raise and lower to get to the right human factors angle. IIRC, standard counter height is 36 inches, so you're a bit low.
I have a maple top workbench with metal frame that is about 2 inches thick. It's good and stable.
Here's a video of the cartridge counter I just made for use with my Hornady lock-n-load press and Lyman taper crimp die: