Geo holdouts and simple lift tests give most businesses 80% of the answer at 5% of the cost.
You do not need a modeling team to know whether a channel is additive. You need a disciplined holdout.
Geo holdouts are the most practical option for regional businesses: pause a channel in matched markets and compare trend deltas.
Run the test long enough to cover a full purchase cycle. Two weeks is almost never enough for considered purchases.
Document the result and re-run quarterly. Incrementality decays as markets and creative change.