most of the time perch do school, but i've found that -as with your experience- double-digit perch often prefer to wander around in smaller "wolf packs" rather than the massive schools the species is known for.
this is purely a theory, but i think these fish are actually hunting smaller perch, minnows, and other large prey rather than seeking out the same food sources that small perch look for; bloodworms, shrimp, etc. in your case, it sounds like they're hunting crayfish, but i'll remind you that perch ARE a baitfish, and perch themselves tend to be heavily cannibalistic. you might try finding one of those schools of smaller fish, and working them with a larger jigging lure like a rapala slab rap or a salmo chubby darter. something in the 2 to 3 inch range. i've had some pretty good success doing that this year.
in my area depth maps aren't really a thing. a couple major lakes are mapped out, but that's it. so i use satellite photos to find interesting characteristics of a lake that i plan to fish.
all that said and done, most lakes just don't have a large population of large perch. you'll catch a hundred under ten inches, and a handful over ten. it takes a really special set of circumstances for a lake to grow a lots of trophy perch.