Which carrier's to hook up with?

Honestly, I'd suggest you just lease to mercer. The thing is, unless you get your own accounts for direct freight you will never make more than a lease guy. I've been down the same road you are on, using load boards for freight and it just doesn't pay enough to cover the extra expenses. I make nearly as much now running under mercers authority as I did under my own. With a lot less work involved, and never worrying about having too run credit cheeks on brokers or waiting for my money.
But that's not what you asked, what you asked is which brokerage outfits to hook up with. Back then I mainly brokered through mercer, landstar, allen lund, bear. And the occasional tql and chrobinson load when they couldn't get a cheap truck.