I ran an agency almost exactly like you're describing and had a lot of success by combining all of your ideas.

I started by hiring contractors to take on the parts of a job I couldn't handle myself. The contractors were put in contact with the client, but under my company's banner. This formed a brand without tying me to huge amounts of salary overhead.

Each project was negotiated separately. This allowed me to plan well, and my contractors had an incentive to work quickly (done in fewer hours meant a higher hourly rate). I charged a markup in the hours my contractors worked.

Later, I hired a few full-time employees to handle certain critical tasks that couldn't be contracted out.

This was so successful that I was able to leave the company entirely and my team still runs without me. I attribute this success in equal parts to honest communication with my team and clients alike, rock solid processes I put in place to create a consistent experience for my team and clients, and finding the best talent by offering good wages and near-total autonomy.

With some solid planning up front, you're looking at huge income potential. I'd love to discuss this further with you if you have questions.

Good luck!