According to the rtings test, you're best bet is 4k at 120hz without VRR.
https://www.rtings.com/tv/reviews/lg/cx-oled
The refresh rate and frame rate are independent. If you tell the xbox to output at 120hz, it will output that regardless of the framerate. To test this, bash the green button on the LG remote. After a number of times, it will display the signal that is receiving and show 120HZ along with other info.