I ended up getting it working last night after a reading a post elsewhere. It seems if you're using the ESIM (on iPhone XS, XS Max, or Xr) that is causing the issue. I swapped from ESIM back to a physical SIM card and was able to activate/link the subscription. Swapped back to ESIM and it's now working.

If however you're not using an ESIM, then I have no suggestion, as I tried literally everything short of wiping and resetting the device, but if you are using ESIM and have a Verizon SIM to swap back to long enough to link with applemusic.com/carrier then you should be good to go.