Parys might be suitable, surface access is no problem at all, however it might not have the feel you're after as the surface areas are long, long abandonned.
The two biggest slate quarries up here, Dinorwic (ceased working 1968) and Penrhyn (working) would both have the problem as you say that you can't get far enough back to take the whole area in. You could photograph Dinorwic from the opposite hillside though.
Oakeley might be suitable, but again that's being worked (untopped, it was an underground quarry) so an approach would need to be made.
my orders are to sit here and watch the world go by