Not a fit
Treat agencies like vendors.
An agency, to you, is a service to procure. You want SLAs, a scope sheet, and someone to escalate to when results aren't there. You'll be happier with a 50-person shop that can swap an account manager onto your account next quarter.
ANTE clients
Treat agencies like teammates.
You want a person, not an account manager. You want someone who'll push back when you're wrong, ship faster when you're right, and treat your win and our win like the same thing.