What is the top metric to follow from a CFO perspective? ROAS? LTV/CAC? Payback time? Unless you connect marketing to a financial statement, it is not easy to get investment.

My strongest recommendation would by Customer Lifetime Value, provided you have the data the calculate it. It is immediately compatible and recognizable to finance. It connects nicely to risk exposure, cost of capital, and facilitates decision making and investment. Return on specific initiatives can be troublesome if you have an attribution problem, which most normal firms do (i.e. inability to properly disentangle several concurrent investments from one measurable outcome in that time period).