@Musician , if you haven’t done so already, have your audi set up a separate program for music performance (I have never used “My Music”. That might be a good starting point).
In that music performance program, have the audi reduce the feedback manager to the minimum that you can get away with without experiencing frequent feedback. That should reduce any “warbling” you might hear while singing high, sustained notes.
Eliminate or minimize all speech comprehension enhancements and compression.
Consider buying some “Hearing aid sweatbands”. These are small moisture-wicking cloth covers that fit over your HAs. Try stuffing some cotton or something into the hearing aid sweatbands to try to muffle the microphone inputs on the HAs. Or think of some other way to muffle the HA mics. Maybe try scotch tape over the HA mics. If one layer of scotch tape isn’t enough, try two or three layers.
As an opera singer you are probably singing really loudly and the HAs are probably distorting right at the microphone input. So your HAs are trying to process sound that is distorted from the very beginning. The key is to reduce the volume of that audio signal (your voice) so the HA mic doesn’t clip the sound wave. Hope this helps. Good luck.