from an engineering perspective, using uprights at 1/3 or 1/4 reduces the sag "in the middle"
I've done billions of 8 ft long x 16" wide shelves using 2x4 @ the 1/3 points. zero failures, and believe me, my DW can pack a mean shelf....
using only 6' lengths, joining them at the center post is a good approach.
question - 102".... a 4x8 sheet gives you 3 pcs 16 x 96 - what's the plan for add the additional 6"
the other thing that will greatly increase the horizontal rail strength is to glue the plywood to the horizontal pcs. the glue bond has very little flex so it's "stiffer" than nails/screws which want to bend a bit under load.