Is there anyway to use a separate Wifi mesh router (eg an Orbi) with the new TMO-G4SE

@GrumpyGogs to your questions.

• From what I can see and have read TMobile app won't let you turn of the router functionality to just use it as a modem.  Correct? Yes. If you have a wifi router then you need to set the network settings to "bridge" mode on your wifi router, then the TM Gateway will spool out IP addresses to your devices. Otherwise, you will have double-NAT issues. 
• I've read some suggestions to use the T-Mobile box as-is and just connect it to an external router using Ethernet.  Anyone here doing this for awhile without issue?  Yes, I am (and others). Don’t things get messed up with the external router and T-Mobile router allocating DHCP addresses? See above - bridge mode for your home wifi router. I use the Eero 6 Plus and set it to bridge mode. Alll good. 
• Saw another suggestion to use the setup above, but rename the T-Mobile Wi-Fi network different than the external router Wi-Fi network and only connect to the external router Wi-Fi.  Any doing this without an issue? Some people have had issues with connecting their IoT devices like thermostats and printers to their TM Gateway, so it seems to be a good practice to setup a 2.4Ghz and 5Ghz SSID separately to make the IoT connections easier for devices. This is my practice and it works well. Also, if you have connection issues, set the 2.4Ghz WAP setting to WPA/WPA2.