I was a bit silly and setup Unraid as a trial as NAS computers are a brand new thing to me and I wasn't sure what OS I was going to use and forgot about it, just used it without thinking. It's been running fine for 3 months and I figured I really should buy a key. I've now bought an Unleashed key, went to activate it and found the USB stick had failed. My pool is setup as ZFS with 2 VDEVs of 4 hard drives and an SSD as a cache drive.

Given I now can't create a backup of the first USB drive, how do I go about setting up the drives again on this new, licensed install key? Will I need to make a new array from scratch or can it rebuild the array if I manually recreate it to be the same configuration?

You can just add a new pool, with 5 slots, assign all the devices together, including cache, don't create subpools, order also doesn't matter, leave the fs to auto, and Unraid will import it at array start.