As long as it's Medicare, I use the G0121 with the Z12.11 as the primary diagnosis. Then I'll list K57.30 or any other incidental findings after the primary Z12.11.
The MAC here is NGS and they process as a screening if the Z12.11 is listed primary (and if the patient hasn't had another colonoscopy in the last 10 years for the G0121).
This is all based on the assumption that there are no other diagnoses to make it a 'diagnostic' versus a 'screening' colonoscopy and that nothing else was done (no biopsies, etc.)
Hope this helps.