Lobster wrote:

How does a DIY'er achieve the ground edges to a glass pane that you get
on commercially-supplied mirrors?

You buy a new pane of glass and have the edges ground by the supplier.
Cheaper and easier all round.

I have a proper wet-disc diamond grinder for doing stained glass work.
For cheap mirror glass (under £50/ m^2 is cheap for the sort of glass
I use this thing on) I'd get the glazier to do it on their big belt
machine.