The trench wall idea is ok,, but pretty skimpy on the 8 inch footings.

If it were me,, and it's not..   I would decide how tall the concrete walls were going to be, what the finished floor grade was going to be at, then pour 14-16 inch footings, build the wall forms (with appropriate rebar) to that finish height. Then,, if the floor needed fill, I would fill and pack those areas of the floor. Then once all the walls were set up, fill in and packed,, then I would pour the floor inside the stem walls, use lots of rebar and 6-8 inches of concrete for the floor. (nothing worse than a concrete shop floor with cracks in it, and trying to roll heavy objects over buckboard flooring..)  I would have 2 feet of exposed concrete wall from the floor up. Reason, very easy to clean even high pressure washer can be used to blast all the gunk out the door and not water log the interior walls.  (neighbor found this out by accident, poured a slab floor for new shop,, then found out equipment was taller than he'd figured and full steel shop had already been ordered,, so he poured a 2 foot stem wall all the way around the outside.)

Just before pouring the footings and stem walls,, I'd also figure out where ALL the utilities were going to enter/exit the shop and get those in. (electricity,phone, water, sewer, etc..) Wouldn't have to put in the actual lines,, but I would get big enough conduit in so electric/etc,, could easily be slipped in later.

Again, that is what I'd do, but you'e not me. ( I would be plenty worried about "several feet of fill" and a skimpy 8 inch wall w/o a good footing..)

Shops are a forever building,, may as well build it right the first time. Not many people say they built their shops too well or too big. Sure are alot thou that wish they'd build it better and/or bigger.