I have a pair of AirPods Pro 2 working with an iPhone 12 Pro Max iOS 16.5. I’ve had no problem at all with it. What iOS is your iPhone on? I assume you’re saying that all 3 of your AirPods (Max, Pro 1 and 2) exhibit the same bad sound issue all at the same time with your iPhone, starting about 6 months ago? If so, then maybe there’s a Bluetooth issue with your iPhone.
Have you tried any other non-AirPods BT device with that same iPhone to see if it still sound bad? This is to confirm or rule out whether it’s a BT issue, or it’s an interaction between the AirPods and the iPhone issue, which possibly relates to the audiogram in the Health app, which I assume you have tried to remove and rebuild?
After you remove the audiogram, but before you rebuild (or reimport) a new audiogram, does it sound bad anymore without the audiogram in place? This is to see whether it’s an audiogram related issue or not.
How did you try to remove and rebuild your audiogram? Did you go so far as to remove/delete the Health app on your iPhone first, re-download and re-install the Health app back to your iPhone from the Apple store? It’s possible that the Health app on your iPhone got corrupted.
Have you done any iOS upgrade/update on your iPhone since? Which iPhone do you have?
Are you able to try to get your 3 AirPods to connect on another iPhone (a friend’s or spouse’s) to see if they exhibit the same issue on another iPhone or not?