One recent discovery is there can be many reasons for cylinder idle issues (outside the normal plugs, coils, injectors and carbon buildup) including PCV valves going bad. Just had this happen to me and I’m waiting on a replacement (just one side). Will report back if this changes my idle characteristics.
We have a semi-local RS4 with over 100,000 imperial miles on it which idles perfectly. And it’s never had a carbon cleaning!
Looking at data via the Ross-Tech VCDS is super helpful as is live logging of cylinder misfires with the same device. Can tell you a lot!