One thing you could try looking at – unfortunately, you’d have to start the copy over is to go into FCP and delete the render files (which can add up very rapidly) to help get that size down to something more manageable. Robert’s suggestion of using Hedge, Silverstack, or Copy That are how I’d probably do this.
Just a quick FYI – Carbon Copy Cloner does not charge a monthly fee. One-time purchase (for current version), with free point updates, and upgrade pricing when/if you want the latest. Definitely not a subscription model, and I dare say a very generous trial mode.
Not affiliated with Bombich, just a long-time satisfied customer.