The simplest way to do exactly what you want is to use LVM to create a linear volume (equivalent to JBOD) from the two 6TB disks. Then create a zpool with a single RAIDz1 vdev with that along with the other 2 12TB disks. You could use mdraid to do a RAID0 as you suggested too. The result would be similar. In fact that could be easier for you if you already know how to use mdraid and you don't LVM.
You could also do it all with ZFS albeit with more lost space. You could create a zpool with 2 vdevs. One with a 6TB mirror comprised of the 2 6TB drives. The other a 12TB mirror. The redundancy in ZFS is at the vdev level. A zpool contains one or more vdevs. It combines their space like a JBOD. You can mix and match the size and type of the vdevs. You can have mirrors with RAIDz, just mirrors, just RAIDz, etc. My suggestion for having two mirrors, 6TB and 12TB results in 18TB usable space. This is straightforward, easy to manage and easy to expand. You just add another vdev to the pool with whatever topology you like. If you want to maximize the space with what you got, you can do your idea instead. It's got a bit more setup and a bit less redundancy but it'll work fine.
EDIT: Ensure your external drives work reliably under load for extended periods! Either load test them with something and/or watch for errors while transferring data to the new zpool. If you see an error, check dmesg for anything related to a USB drive. If you encounter such a problem it might be a controller bug, controller overheating or a bad cable. Controller refers to the enclosure SATA-to-USB bridge. I have 5x WD Elements, one WD MyBook and 2x NexStat 3.1. They're all using ASMedia. The WD Elements are prone to overheating if there's no appropriate ventilation. I've had to adhere tiny heatsinks to two of them in order to resolve overheating as they operate at higher ambient temperature. Crucially all of this overheating has occurred under load. Without loading them, it's all looks fine and dandy. ZFS did not lose me any data when any of this happened and as I addressed it.