My shop is the three bay garage I built when I tore the old one down.
My height limitations (like yours) came from the facts that I have an entrance to the building above onto the roof of the garage and it's got to have a flat roof and I wanted to pour a recreational deck on top.

I maximized the head room by using a 19 KIPS steel I beam to span the' length and 2*10 rafters making the 20' span with the beam in the middle The 2*10s set on the lower shelf of the "I" beam.

I haven't insulated the roof but if I do it'll be in between the rafters and that's it. I may use that foamed board because it's got a reflective face and won't bleed raw fiberglass all over the place.

When you wire the place you gotta leave access to all your junction boxes.