Nick,
It seems like you have 2 unrelated issues. The first is that your Cortado will not bias low enough. This is because the internal gain of some of the FETs is high enough that they need a little more than the 5K of the trim pot to get the bias down to .075V. The .1V that you are able to bias to is perfectly fine. The main issue in this circuit is that the FETs are matched to get the lowest common mode rejection.
The second issue is causing the scratchy bursts of sound. This is usually caused by low phantom power (use your meter to make sure you’re getting 48v from your mixer) or the one side of the balanced configuration of this circuit becoming shorted somewhere. Make sure no part of the piezo disc is shorted to the shielding…continuity test between the 3 wires coming from the piezo disc cable. Also check the XLR jack and make sure nothing is wired backwards or shorted. While you’re at it, double check all your solder joints; you might want to re-flow them if they look suspicious.
Let me know if you have any more questions.
Good luck.
-Brach