Interesting question.
Has that lake been stocked with walleye?
If not, the small walleye have evolved to hang out somewhere you're not fishing, to avoid predation. I bet if you threw small jigs after dark (when walleye have the advantage) in the most walleye'y spot you know on that lake, you'd get a couple small pics.
If it was in my budget would have 4 16 Lowrance graphs . Don't knock it till you try it ! Currently run a 10 and a 8 not bad then used a boat recently with 12s HDS live iam sold old eyes.see it better lol.
As for the 20ft deep remark maybe here in Ontario over 20 ft bass fishing isn't overly common but try staying on ledges and sunken timber in 60-80 ft of water in southern USA reservoirs all this technology gets developed in conjunction with pro s and then it's a trickle down effect to us weekend warriors