Spend on your biggest channel until it hits your ROAS floor, then add the next

When optimizing for maximum spend under a ROAS constraint: max out the biggest channel until it hits the ROAS threshold, then move to channel two until it hits the threshold, then channel three. Only add a channel when existing ones are saturated or it creates an interaction effect (e.g. brand channels). A new channel must exceed, not match, the ROAS existing channels would get on the same budget, because it adds integration, creative-format and measurement overhead. High ROAS at tiny spend is not a reason to add it.