You didn't say whether there is active ground water at or above your ftg elevation currently. If there isn't then you have less of a concern.
2' of rock above the drain tile is plenty. If you want to go higher with rock consider drain board on the fdn walls, certainly less labor and equipment cost than filling with rock.
What is the native soil? Unless it's clay you can use it to backfill even semi structural areas, garage slab, drivepad, etc as long as it's proper moisture and compaction. Think of it this way. Your entire house is sitting on a 2-3' wide ftg on that material (unless you sub exc'ed and bridged over the native material).
IMO the reasons most residential dirt guys push using structural fill everywhere are:
They make more by selling you fill and potentially hauling off the extra material. They make more by charging full labor rate to place and compact when placing granular material requires much less compactive effort and moisture control. They limit their liability for settlement when they don't do the above properly and you garage slab cracks and settles.
"premium fill" I'm guessing is not 100% granular material in your case, rather it's material that has a better R value than the native material there. You need to decide where that is absolutely necessary and where you can get away with native. Good example, use premium fill at the front of the garage where the subgrade will get continually pounded by your vehicles pulling in and out. But around the other 3 sides, how much load will the slab see?? Some tool boxes and workbenches is all. Maybe 1 side where the boat gets parked up close to the wall?
'06 X2 MCX
"I understand why some people may not want to do this the way I have recommended but I can't understand the death grip some people have on a toilet plunger with a hose fitting." -JimN