Following because I ran into this issue too on my early edition iPhone 12 Pro. With WiFI disabled from control center, the phone does not panic. With WiFi enabled, iPhone panics within ~3 minutes. I see the same error logs as you: "no successful checkins from wifid in 180 seconds" when the panic occurs. No issue like this prior to 15.4.1 while on 15.3.x. Going to try 15.5 and setup as new phone, and slowly add apps back.