Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:

6 acronyms in this thread; the most compressed thread commented on today has 30 acronyms.

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I believe ZFS works best when having direct access to the disks, so having a md underlying it is not best practice. Not sure how well ZFS handles external disks, but that is something to consider. As for the drive sizes and redundancy, each type should have its own vdev. So you should be looking at a vdev of the 2x6TB in mirror and a vdev of the 2x12TB in mirror for maximum redundancy against drive failure, totaling 18TB usable in your pool. Later on if you need to add more space you can create new vdevs and add them to the pool.

If you're not worried about redundancy, then you could bypass ZFS and just setup a RAID-0 through mdadm or add the disks to a LVM VG to use all the capacity, but remember that you might lose the whole volume if a disk dies. Keep in mind that this would include accidentally unplugging an external disk.

You’ve got some decent answers already, but since you’re getting interested in ZFS, I wanted to make sure you know about discourse.practicalzfs.com. It’s the successor to the ZFS subreddit and it’s a great place to get expert advice.

Is there any particular reason you're interested in using ZFS?

How do you intend to move over the data on the 2x6 array if you create a new pool with all the drives?

mdadm RAID1 is easy to handle, fairly safe from write holes and easy to upgrade.

If it were me I'd upgrade to a 2x12 array (mdadm RAID1 or ZFS mirror, whichever you want), stored internally. And use the old 6 TB drives as cold storage external backups with Borg Backup. Not necessarily for the media files but you must have some important data that your don't want to lose (passwords, 2FA codes, emails, phone photos etc.)

I wouldn't trust USB-connected arrays much. Most USB enclosures and adapters aren't designed for 24/7 connectivity, and arrays (especially ZFS) are sensitive to the slightest error. Mixing USB drives with any ZFS pool is a recipe for headache IMO.

I could accept using the 2x6 as a RAID1 or mirror by themselves but that's it. Don't mix them with the internal drives.

Not that there's much you could do with that drive setup, since the sizes are mismatched. You could try unraid or snapraid+mergerfs which can do parity with mismatched drives but it's meh.

Oh and never use RAID0 as the bottom layer of anything, when a drive breaks you lose everything.

Make sure you understand volume block size before you start using it. It has a big impact on compression, performance and even disk utilization. In certain configurations you may be surprised to find out as much as 25% of your disk space (in addition to parity) is effectively gone and it is untrivial to change the block size after the fact.

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.