Google Fi leaves DNS data on the iPhone when you use an eSIM or physical SIM card. The ONLY way to get green bubbles SMS within group message to send within iMessage is to clear this DNS.
The ONLY way to do this is NOT to RESET NETWORK SETTINGS but to RESET ALL THE SETTINGS(see below). Can reset the entire phone if you want but if you do this DO NOT use prior backups as the DNS could be in that old backup.
Also ensure your Google Messages CHAT FEATURE is off BEFORE you remove the SIM card /eSim and port out your number, so you number isn't locked to SMS within Google Messages. If you didn't do this then you could google Deactivate Google Messages and enter your phone number. Problem is that it will send you a basic SMS text to verify its you. If you can turn the eSim or insert the Fi Card back into the phone. Once this is done you may be able to get the SMS text.---maybe. Depending on how far the porting process is.
If you have your Google Fi SIM card:
Eject your Verizon SIM and insert your Google Fi SIM.
Go to Settings > Cellular > Cellular Data Network (this menu is not visible if your Verizon SIM card is installed 🙄)
Scroll to the bottom and hit "Reset Settings"
Eject your Google Fi SIM and reinsert your Verizon SIM.
Reboot your device.
That should do it, provided you have MMS turned on in your Messages settings.
If you don't have your Google Fi SIM card:
I believe your only option is to reset all settings, since this options menu is simply not visible without your old SIM card. This will reset all customized settings, so take a second to note any settings you'll want to keep as they are.
Go to Settings > General > Reset > Reset All Settings.