OK, when you put the basic piezo gyro on, did you use its remote gain function? Or were you able to disable the TX remote gain and set the gain manually with this basic piezo gyro?
WHat kind of basic piezo gyro are we talking about?

Obviously I am thinking that the gain on your gyro isn't being set correctly by your TX somehow.
And you have evidently gone thru the heli mechanical setup very thoroughly so that probably isn't the problem.

So let's get a gyro on there that can be set manually, disable the TX gain function, and see what happens.

Also be sure that you are using the very shortest possible servo arm geometry, link rod in the hole closest to the servo axis for these tests.