I think there is a big possibility that the problem is on iPhone. I have an iOS 14.1 iPhone 12, and there is static during phone call and music streaming when connected to my AirPods.
I have Phonak Marvels with an iPhone 12 and I was having static issues on phone calls. I am not sure how the Costco hearing aids work with bluetooth but it was suggested somewhere that I try connecting to Headphone instead of Hearing aid. This seems to have sorted the static issue. To do this I go into bluetooth and select my R hearing aid - click on the i and then Device type - select headphone.