1mm...? we use only 3mm, and keep a sheet or two of the 2000 x 600 size engraver's brass on hand.
My experience with thinner stuff is the headaches & gremlins that sneak in are not worth the minor substrate cost saving- besides a plaque in real brass, as opposed to gold-coloured aluminium, looks better with some gutsy depth to it.
Chrome plating the plaque afterwards? yuk!
We chrome plate brass horse harness elements, but a plaque is different. They're meant to be read - a message understood - as opposed to being 'pretty bling'.