Okay, I see where you're coming from. I have too many detectors including several with displays. I've never used the display in quite the way you are thinking, but have found it helpful separating trash from likely coins when deciding to dig in well kept grassy areas. I don't normally pay much attention to the depth read out because if I get a solid coin reading, I will retrieve the target regardless of depth. As I mentioned above, depths below 6 - 8 inches the display may not be accurate as to the depth reading or the coin versus trash indication.
On my Vaq and other beep and dig detectors, I have found that with a bit of practice, thumbing the discrimination gives me nearly as good an indication of coin versus trash as the display, just takes a bit longer. Only problem, it's common for many detectors to lose a bit of depth at full discrimination which is not a help when trying to decide on a deep target which puts us back into all metal which just tells us the target is metal.
In my experience I haven't seen any method that is totally reliable to avoid digging some trash. You mention digging on a sound you wouldn't normally dig. I've retrieved a fair number of good targets that just didn't fit either what I'd call my normal solid good sound, nor did they quite match my normal trash sound. I've become a believer in dig any sound you wonder about as it's the only way to know for sure. You'll end up with more trash, but you'll end up with a few more desirable targets as well. As someone has mentioned, "the only sure discrimination is a shovel".
BB