Why not piles under the bent posts with a thickened slab perimeter say to 18" deep ( or whatever it takes for any frost issues.You can backfill the building lower this way to maintain a concrete perimeter wall keeping the walls/ wood portion of the building 12" above grade this way. Or pour a small grade beam first. What we do is pour a small grade beam instead of the thickened slab. Do the frame and close in the building. Then pour the slab inside. You have weather protection when you pour that nice pad. Also, the slab is poured lower than the top of grade beam so you have your wood off the ground inside and out. Just remember to buck out the grade beam for your doors. Add a few inches to.the side R.O. For trim or overhead door backing.