Designing our first CTV test post Series A, incrementality testing or trust the platform's numbers?

First real test of streaming TV advertising is being planned for the fourth quarter, since we've just completed our Series A funding and the entire venture depends on whether the design is good enough for the TV ad attribution to yield any results by the end of the experiment. The choices are a geo holdout, a simple before and after comparison, or simply relying on the platform's reported conversion figures and making adjustments later. That third option is the one the vendors are pushing us towards which says something. What our data specialist prefers is a geo split which means fewer markets