Rear suspension bottoming out on rough roads.... where to start?

There's no spacers in there currently, but its not a bad call, spacing up the back end to level it out and then go from there. Cheers.

30mm lift in the rear is roughly what you get if you put front springs in the rear, which in itself isn't a bad mod but it's no good if you need more clearance. I'd ditch the Pro Comp shocks straight away. They are universal fit designed for much heavier vehicles than the Jimny and always result in an unpleasant ride.