You'll want a layer of foam over each exposed metal member, otherwise those cold beams will still sweat inside.
The idea of leaving a gap and securing directly to your framing is a good one, but the air space will act more like a "rain screen" and won't necessarily make the wall better insulated.
Since you have surface mounted electrical, I'd stick to your plan of going between beams (and overlapping a second layer over each beam). Later on you can add another layer and a few years later another layer... Etc.
Don't bother with the interior vapor barrier. The foil facing of the foam will act as one already (usually) and having the ability to diffuse any random moisture is still a good thing. Some sort of durable wall board would be a good idea to protect the foam boards. Plywood, osb, hardboard, drywall, concrete board, doesn't matter a ton, but any all-wood materials are at greater risk for fire than drywall (which actually produces moisture/water as it gets heated) or concrete board.
Since drywall sucks up standing water and is easily damaged, I like thin cement board for the first 4' or so of a garage wall.. But that's just me.
Another option would be to layer metal siding on the inside.. Durable, easy to replace, no mud or mortar to install it and can be taken down to augment your insulation in the future. Also less prone to fire/burning. Even corrugated metal roof panels could be used, even if only partially sheathing the wall.
Just my $.02
