I have been using the Tandem X2 and xDrip+ with the G6. The only out-of-the-ordinary thing I am doing is using a Sony watch as the xDrip+ bridge to receive G6 readings and relay them to my phone.
The Tandem seems to replace all functions of the G6 receiver without any differences that I can see, so the two are interchangeable.
I don’t restart my Dexcom sensors any more because I am lucky to get 8 days out of the current crop of sensors. Back in the days when I did restart with xDrip+ I generally got 11 to 12 days or a little more, and one exceptional one lasted 22 days. Something has changed.
I have restarted two transmitters that were older but still communicating using the xDrip+ instructions. For that I changed the transmitter ID in xDrip+ to do the restart and left the pump alone. But I have had problems getting back to the working transmitter on xDrip+ and can’t remember how I resolved them. Think maybe I had to switch to the phone as the bridge for G6 readings. Somebody else may have better advice for that. A search on this forum for restart will find instructions.