Update: went to the local Verizon store for help. They have seen custom ROMs like GOS and didn't freak out, which surprised me. They went through the same carrier reset steps, permissions changes, default app settings, etc outlined in the GOS forums. No joy. As before, I could text them immediately but never received their replies. I asked if they thought something might be wrong with my account or at the network level. They said no because I can send & receive voice calls, get on the internet, and at least send texts.
So when I got back home, I did a GOS reload via Settings > System > reset Options > Erase All Data. GOS came up in the default state, I clicked through the usual setup options, and was able to place & receive voice calls. I can send texts... but still do not receive a reply.
This makes me think there has to be something wrong at the Verizon account or network level. How could a freshly reloaded GOS installation not receive texts, while everything else works?
I had already been thinking about upgrading to a Pixel 9, but if the problem is at the account/network level, a new phone won't fix the problem.