I think the issue then is that setting in Android Messages > Advanced > Phone Number is misleading.
I think you and the OP are assuming that is should let you specify the phone number to send messages from when it appears that it's really just there so that you can see what your number is. I don't think that field should be editable.
Right. That feature is misleading. All these people are being unreasonable, and their mistake is in expecting that field to do something, when in reality it does nothing. Why on Earth are these whiners unable to wrap their heads around that one?
You're saying these users should use the Google Voice app or Hangouts for SMS, and stop trying to make Messages do something it was never designed to do. Well, OK, but if it wasn't designed to do that, why is it what Google recommended users switch to when they jerked SMS support from Hangouts?
You also say that Messages ties into the local phone SMS database and has no cloud component, and that Hangouts and Google Voice don't deal with local messages. Well, when I use Messages for web, in a Google Chrome browser that is logged into my Google account, I can see all my carrier SMS conversations that took place before I was using Messages. Boom, my local SMS messages are officially in the cloud. Just not the ones sent to/from my Google Voice number. Google syncs local data between my Android apps and my Google account in the cloud all the time. Look at Drive, Photos, Play Music, etc., etc. etc. Why is it unreasonable to think that if Google is recommending a replacement app for the SMS functionality within Hangouts, that it might function the same way and put SMS messages sent to/from Google Voice into the local SMS database on my phone, where Messages can display them, and use the number I enter in the Messages app to send messages?
As others here have noted, when any app on your phone INCLUDING GOOGLE ASSISTANT tries to send an SMS from your your device, the list of applications you can choose from to perform that function are limited to whatever SMS apps you have installed (in my case, LG's stupid app and Google Messages). Even on pure, carrier-agnostic, vanilla Android, running nothing but apps that are part of what's often hilariously called Google's "ecosystem," your Google apps are too stupid to leverage each other's capabilities and leave you with a crippled device. Google Assistant can't send an SMS from a Google branded phone using Google Voice. It used to work from Google Hangouts, but they inexplicably nixed the ability to set Hangouts as your default SMS app, even though it still sends and receives SMS messages for Voice users. Google told people to install Google Messages instead, and that's not a solution for anyone using Google Voice. What was possible before isn't now. How is that not the same thing as Google breaking something? How is it the fault of these users for wanting a solution that works?
Some of us like to have a dedicated messaging app, that doesn't require one to open an ugly app like Google Voice and switch to a messaging tab. Some of us like to see individual notification badges on app icons, showing the count of new SMS messages separate from the count of new missed calls or voicemails. Most people can do this, but as usual, Google Voice users are left twisting in the wind. But even if you consider those issues purely cosmetic, the fact that apps running on the phones of Google Voice users have absolutely no way of sending an SMS from a Google Voice number is most assuredly not just a cosmetic issue or a case of users being too stubborn to use the intended app. There is no intended app that works in this capacity.
The reason that Google is so infuriating is that they spin forward-thinking ideas into wonderfully useful tools, but they either stop developing them before the full vision is realized (Google Voice), or change direction and pull features many people depend on, before they've gotten a complete alternative in place (Hangouts), so the people who get punished are the ones who buy in with the most loyalty. The people who experience the problems are absolutely right to be vocal about it, because they're Google's most devoted users. In this case, the solution is to not be a Google Voice user. How is that acceptable? Google Voice says it supports SMS, but that's only partially true. You can send/receive SMS messages from a crappy tab in a crappy app, but as far as your phone is concerned at the system level, Google Voice can't be used for SMS. It would be one thing if people were warned of this before they incur expenses porting their number to Google Voice. Before MMS was supported, they warned people that MMS only worked via email. I waited until they had everything sorted out via Hangouts to port my number, and then they broke it. Now a couple years have gone by, and there's still no apparent solution.