First, is this a supported ECM? Checking quickly, L83= older gen, but need to confirm. 2019 was the start of the new-gen trucks with encrypted ECMs, if that's used your ECM may not be supported by EFILive at all. 6L80's all use T43 IIRC, so that will likely be supported.

Post your ECM and TCM tune files.

You'll need to change the shift points in the transmission as well, just changing the rear end ratio will throw shift points off and it'll behave oddly.

VATS is a "patch", to remove VATS you need to find the parameters in the tune file. I don't have an E92 in front of me, so the exact location may be different. System -> Parameters -> VATS. Engine Operations -> Operating System Patches -> Parameters and find the VATS Disable patch and enable it. You will need to perform a Full Flash instead of Calibration Flash for these changes.

Rear O2s may/may not be available, they're emissions components even in a swap application.