Shimming starters is a genuine pain in the asp.
There's several different methods of determining the so-called "proper" shim thickness; and even the instructions that come with the starter are generally inadequate.
I've seen starters that needed a shim under one bolt but not the other. Under the inner bolt, but not the outer bolt. Under the outer bolt but not the inner bolt. One shim under both bolts, but a second shim under one bolt.
And don't get me started on flexplates that aren't actually ROUND (or on-center) at the ring gear. Or the crank-grinder who doesn't properly center the crank in the grinder when he takes that ten-thou off the main journals, and now the flywheel flange at the rear is no longer concentric with the main saddles. WHO is going to dial-indicate the ring-gear to look for runout, and what would you use as a measuring point on the thing anyway? The possibilities for screwups is so great that it just boggles the mind. It's amazing that engines work as well as they do.
And then, there's folks using the wrong bolts to hold the starter to the block. Both bolts HAVE to be knurled; but mini-starters need bolts that are stepped at or near the knurl. The hole in the mini-starter nose is generally cast--not machined, 'cause that costs more money--and typically 10mm nominal diameter. Bigger than the old "3/8" bolts, and absolutely larger than the 3/8-16 threaded hole in the block. So even if the bolts thread into the block, if they aren't stepped down from 10mm shanks to the 3/8 thread, they may still allow the starter to shift under load.
If you have the old direct-drive heavy-ass starter motor, you NEED a brace at the front, and dirtbags tend to throw them away at starter changes on customer vehicles. Last I looked, GM was still selling the braces for SBC and BBC, but not for Olds/Pontiac/Buick/Cadillac. If you have a modern GM PMGR "mini-starter", they--surprisingly--tend to require fewer shims than the old units; but that doesn't change a misaligned block/crankshaft. And the aftermarket PMGR starters may or may not be as accurately made as the Genuine GM original-equipment starters. TORQUE the bolts as that will help prevent the starter from moving around on potentially-imperfect holes.
I don't have a good answer for you. Try some other shim combinations, see if you stumble on something that works better than what you have now. I need to do exactly that with my '97 K2500 7.4L with an aftermarket "GM-clone" PMGR starter motor.