You haven’t provided any info about your partition scheme for either drive, but I assume you’ve got your bootloader installed in an EFI partition in the newer drive. You will still have an EFI partition on the old drive created by the Ubuntu installer, so just be sure you know which bootloader you’re using.
Yes, the new drive has a boot partition mounted to /boot/efi, according to the Disks utility.
It’s not clear what issues you’re worried about, but if you’re nervous about breaking the Ubuntu installation[…]
Actually, that’s a good point. I’m expecting to get rid of the installation anyway, so I don’t need to worry about breaking anything there.
It’s not clear to me what the goal of option 3 is
Same as option 2, avoiding breaking a system I’m getting rid of anyway.
Thanks for pointing out the errors in my line of thought!