[unknown] Yeah, it was stuck on verifying, so I uninstalled it and did the old method of installing the outdated beta version, manually setting my ACS url, then updating to the latest app version, and that made it connect immediately. In the past, when I enabled RCS with that method, it disconnected after 3 days, but I'm hoping that the added change of enabling Sandboxed Google Play > Confirm number with Google (which I am just going to leave toggled ON, just to be safe) will be what is required to make it stable. I am on Verizon, and I suspect it is Verizon's implementation of RCS that is less stable than other carriers'.
I'll report back in 4 days about whether it worked or not.