Use an Ubuntu live USB, all recent versions of Ubuntu have ZFS drivers baked into the live environment. Then you should add your new SSD to the ZFS pool, and remove the old one from the ZFS pool. Your m.2 WiFi slot should be able to host the 2nd drive while you do this, but if not you can use an external USB housing for it, you'll just have to make sure that the ZFS pool knows its UUID so that it knows it's the same drive.

If you want an image, it doesn’t matter what the underlying file system is. You should be able to use a tool like Clonezilla and get a 1:1 copy. Depending how you’ve set up partitioning, you could also use sgdisk to set up the proper partitions and zfs send/recv for the new data portion of the drive and install a boot loader. That’s probably the way I’d go in this instance.

don't mess around with imaging to a file on the zfs, then restoring it. simply clone nvme -> nvme using a usb nvme adapter then replace the internal with the clone.

It sounds like nobody actually understood what you want.

You have a non-ZFS boot drive, and a big ZFS pool, and you want to save an image of the boot drive to the pool, as a backup for the boot drive.

I guess you don't want to image the drive while booted off it, because that could produce an image that isn't fully self-consistent. So then the problem is getting at the pool from something other than the system you have.

I think what you need to do is find something else you can boot that supports ZFS. I think the Ubuntu live images will do it. If not, you can try something like re-installing the setup you have, but onto a USB drive.

Then you have to boot to that and zfs import your pool. ZFS is pretty smart so it should just auto-detect the pool structure and where it wants to be mounted, and you can mount it. Don't do a ZFS feature upgrade on the pool though, or the other system might not understand it. It's also possible your live kernel might not have a new enough ZFS to understand the features your pool uses, and you might need to find a newer one.

Then once the pool is mounted you should be able to dd your boot drive block device to a file on the pool.

If you can't get this to work, you can try using a non-ZFS-speaking live Linux and dding your image to somewhere on the network big enough to hold it, which you may or may not have, and then booting the system and copying back from there to the pool.

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

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I think you want to see "zfs import" command, to get your pool back con the new OS.

You could use something like Veeam Endpoint (free), it will image while the OS is running so the underlying ZFS system shouldn't matter.