It sounds like your workflow is already fairly efficient, all things considered, if 1) mailing media cards isn’t an option, and 2) you want to do all the color and audio.
Just thinking this through out-loud, but mailing the cards (or paying for high-speed file transfer solutions) would off-load everything to the remote editor with only a final to come back to you for review and posting. That would probably triple (or more?) the amount of time and storage they devote to the projects. You’d also have to trust their audio and color work.
Or, they just send the final back for your color and audio pass before posting.
A proxy workflow, I think, sounds like it really would only address the size of hand-off footage. That might make some parts of this efficient, but there’s still the same amount of time devoted to off-loading the cards, storage organization, editing, color, audio. And then add on top of that time to create proxies.
If you went to Premiere, you could at least send them proxies, you keep the raw, and all that you send back and forth is the project file (once they have the proxies). Then you could work on color and audio while they work on the edit, and then you export the final. Just be sure to so some homework with a proxy workflow, and run a couple of tests. But, that “might” get you a file transfer less than the ~13 GB sent now.
Again, just thinking out loud here . . .
— Joby.