As far as I know, streaming music from iphone stuttering issue is actually an iphone issue, it's getting less frequent now with ios 18.6.2, 18.6.1 was a disaster.
If it's a gen2, it sounds like a wifi connectivity issue that some gen2 have. I've gone through multiple troubleshooting with the service and finally give up.
Having the same issue on my gen2 since day 1. It's getting worse now because of the sleep "improvement". The only way around is to turn off the wifi and use LTE only.
I worked directly with Chase hoping for a smooth transition and shorted turn around time, ending up buying it out within 2 months at 7.5% APR. Rate isn't ideal but I can re-finance it anytime when the rate is better.
I have a gen2 R1S and have the exact issue starting on day 1. It has been 2+ weeks since I reported it to Rivian service and their remote diagnostics team is still looking at it... This is definitely not normal, it should be sleeping when not driven, not offline.