How to implement multi-touch attribution for holistic ROAS on Google and Meta?

[JacobJ]

I manage digital marketing for a mid-sized e-commerce brand, and while our overall revenue is growing, I'm under pressure from leadership to prove the efficiency of our ad spend across Google and Meta. Our current reporting shows decent click-through rates, but I suspect our true return on ad spend is being inflated by last-click attribution, which ignores our strong organic and email channels. For other marketers facing this, how have you successfully moved towards a more holistic measurement of return on ad spend? What specific multi-touch attribution models or incrementality testing methods did you implement to get a clearer picture, and what tools or platforms did you find most practical for a team without a dedicated data science department?