In my experience there's a couple of ways you can go about this. For one, you could go talk to an agency and start working for them on a freelance/project basis. You won't have to take a pay cut and will be able to work closely with a bigger team. On the other hand, you'll also have to deal with the fact that you probably won't have a lot to say in the project, as there will usually be an internal project manager / creative director.

The other way to go (which is what I'm doing) is finding a couple of other freelancers with a different focus than yourself and teaming up with them on a per-project basis. You'll be able to tackle bigger projects in a flexible team and a flat hierarchy. But you'll have to find a co-working space or work remotely and you're less likely to get really big jobs at first because large corporations don't tend to trust freelancers with their biggest projects.

One thing I would advise you to think about as well is that you should be pretty well set up financially (or at least have a pretty good plan) before taking on large (6+ months) projects. Even with down-payments, there will be months at a time where you will be swamped with work without a paycheck to look forward to. Big projects are super exciting to work on, but are a pain in the ass to handle, plan and complete. Always keep that in mind.