I'm going to play devil's advocate here and say I think you should consider approaching the agencies.
The existing agencies likely have very strong contracts with the companies. There may not be the opportunity for you to get in with the company until the agency contract comes up for review. If you don't offer full agency service, you may not be able to get a foot in the door even then.
Even if you can get in to talk with the company and you can find a niche within their marketing that isn't covered by an existing agency contract, the agencies are sure to find out about you talking to their client. No matter how much you protest your service isn't designed to replace theirs, they won't see it that way. Competition in that field is cutthroat, and they won't hesitate to crush you if they think you're trying to steal a major corporate client.
On the other hand, if you approach the agency from a position of a subcontractor, you could get not only their (positive) attention, but their cooperation. You'll have to price your service in a way that they can afford to tack their profit margin on top, but in return you get "friendly" access to not only the one or two clients you might have intended to approach, but potentially all
the agency's clients.
Make one sale (to the agency) and get access to multiple clients. That's the way to go, in my book. They "sell" the client on the service, you provide the service in the background, they bill the client, take their cut and you get the rest. Easy-peasy, and saves you the headaches of client recruitment and account management.
, less taxes, adjusted for inflation.