If you look at your Phone info screen now (* # * # 4636 # * # *), at the Voice Network Type line, I'd be curious what it says (and it would be really interesting to know back on stock, but too late for that). In September, on a working LOS, mine said "1xRTT," which I'm reading is a technology associated with 3G and CDMA (and something that's now gone). That probably explains why the line reads "Unknown" today, because without VoLTE it's stymied. I imagine yours says something else on that line, but is it what it should be given that yours is not completely working yet?
Update: Just to address some of my own questions, for "fun" today I went back to stock, because last year when I was on stock I had already moved to LOS before ever properly provisioning the phone., explaining why last year the voice part was using 1xRTT.
Anyway, that went well:
-VoLTE was Provisioned and enabled (and also appears as a live toggle in the main Verizon area in Settings)
-"Mobile voice network type" was LTE, as was the data network type, answering the above question.
-IMS Status was Registered
-Calling and texting was working fully.
Then I went from there to the final 19.1 version and as expected all things continued to be well, and there was even one improvement, as "Wifi Calling Provisioned" went to Enabled.
On Tuesday, I'll go to the new build of 20 and expect to hit the wall then...but who knows, maybe not.
Update 2: I have now upgraded to 20 (20230117) and can confirm the earlier findings mentioned up the thread. Outgoing texts are the only thing that stopped working despite all indicators looking exactly as they did in 19.1 (VoLTE on, etc), when everything was fully working. The stock text program says "Can't send message with Verizon. Error 0." while Google's says "Can't send. This recipient's phone number can't be confirmed. Error code 0."
Update 3: A workaround, of sorts, is to use RCS instead of SMS (RCS uses a different system than SMS and is more like chat). Google Messages, for example, supports RCS when you're fully opted in to its "Chat features" function, as seen in its Settings. Not everyone supports RCS though on the other end, but it's better than 100% of your texts failing.