The "Lancet Surveys" were very controversial : https://en.wikipedia.org/wiki/Lancet_surveys_of_Iraq_War_cas.... Numerous critics doubt the results.

That's true in part. But if you gloss over (or understand & accept) those internal differences when planning the invasion, then really the consequences are largely also on you. Every middle-east expert would've predicted the region would be destabilised leading to high-odds of (civil) wars for many years.

I think a Middle East expert with have said the invasion would very likely incite significant violence and economic harm. However, they would have also said a civil war in the next 20 years even without an invasion was also a significant probability. So, plotting out the number of excess deaths assuming a completely stable Iraq is overly optimistic thinking.

As external observers it’s easy to think of Iraq as a horrific civil war. Yet, it maintained positive population growth so that’s not what happened. It’s more accurate to consider things in terms of Mexican gangs and lack of social order than civil unrest.

But don't forget the context how Iraq became like it was before the invasion.