Buy the thinking and the doing from different people
If the team that spends your media budget also writes the report on how the budget performed, you do not have performance reporting. You have a self assessment.
Most mid-market businesses buy strategy and execution from the same supplier. It is convenient, it is cheaper on paper, and it removes a handover. It also creates a conflict that nobody names.
The conflict
An agency running your paid media is measured on the results of that paid media. It also produces the report on those results. Even with complete good faith, and most agencies have it, the definitions that get chosen will tend to be the ones that show the work in a reasonable light. Nobody is falsifying anything. The framing is simply not neutral, and it cannot be.
What separating them buys you
When measurement and strategy sit outside the execution relationship, three things change. The definition of success gets set before the spend rather than after. Underperformance surfaces at the point it starts rather than at renewal. And the conversation about whether to keep funding a channel becomes evidence based rather than relationship based.
It does not mean firing anyone
This is not an argument for changing agencies. Good execution is worth paying for and switching is expensive. It is an argument for buying the judgement layer somewhere else, so that the people doing excellent work have someone independent confirming it, and the people doing weak work cannot hide in the reporting.
The practical version
Keep your execution where it is. Put measurement, reporting and budget allocation with someone who has no stake in which channel wins. Then let the numbers decide, and be prepared for them to decide something inconvenient.