Option 1 is what I would do. Seems very straight forward. And additionally:
Moving files around or deleting files is a very quick operation, because the filesystem will only edit the “metadata” and not have to copy the whole file bit by bit to a different disk.
Maybe delete the OS files first (everything except /home). So it’s not cluttered from the start. Then copy stuff from /home/user/stuff to /
Take special attention to the /boot partition. I don’t know which drive the bootloader for your nobora os is installed. It may have been automatically put together with your Ubuntu to your HDD.