Phew! I almost believed I was asking for something beyond the scope of linux-fu. English not being my first language may be part of the reason but still I think I covered everything that was relevant.
Yes, that's exactly what I want and your post has given me the clarity I needed. M.2 wifi slots don't support disks so that option is definitely out. I'm going to boot with the latest Ubuntu live OS on a USB and attempt what you've outlined.
I don't have anything really critical on the zfs that is not backed up separately so I'm definitely going to attempt this and learn in the process.