If I am following the unlocked post correctly then the model number for the S8 should be the G965U1. But when I look up that model number on GSMArena it is tied to the S9. Might be a formatting issue on the table.
See the models/versions listed in the S9+ specs to confirm G965U:
See models/versions listed in the S8 specs to confirm G950U:
Assuming there is a minor formatting issue with the table then this confirms I need the G950U1 model number for the "USA Unlocked" version of the S8 to be compatible with VZW. Not sure what the "1" on the end of the model number is referring to but the G950U part matches up with my previous research. Atleast I know that the S8 does support the bands that I need to communicate with the Network Extender where I work (13, 66 and 46) so there is a chance it will work on a VZW account.
I'm not as concerned about HD voice or VoLTE at this time. Getting any kind of voice or data reception will be an improvement from having to walk up the stairs to answer phone calls or check voicemails.
You also mentioned that CDMA (3G) is going away at the end of 2019:
This should only impact CDMA phone activations. The Network Extender already offers LTE over band 13. The S8 is GSM/HSPA/LTE and see band 13 so I would think I would be fine. Though, I might have a problem if I choose a 3rd party VZW carrier who caps me at 2GB of LTE then bumps me down to 3G/2G for any overages. The Network Extender may not be able to provide the appropriate 3G/2G data connection after I get cut off from LTE.
All of the 3rd party VZW providers I have looked at list thier VZW network as CDMA right now. That must be for 3G reasons. I hope all that is important is that they are authorized VZW lines and that my extender will service them over whatever the phone supports at the time.
Let me know if you can think of any other snags with my plan.