Copy macOS installation to second partition
Well, I managed to achieve what I needed using the following procedure:


  1.) sudo -i
  Enter root shell
  
  2.) mkdir /Volumes/Storage\ 2/Data/ &amp;&amp; mv /Volumes/Storage\ 2/* /Volumes/Storage\ 2/Data/ &amp;&amp; mv /Volumes/Storage\ 2/.* /Volumes/Storage\ 2/Data/
  This relocated all my data on the target volume to a separate folder
  
  3.) ditto -X -V --nocache / /Volumes/Storage\ 2/
  This copied all files and symlinks from my boot volume to the volume "Storage 2", and made "Storage 2" bootable in the process.
  
  4.) reboot > Storage 2
  Booted into Storage 2
  
  5.) diskutil eraseVolume "Free Space" Macintosh\ HD disk0s4
  Deleting the old boot volume, and leaving free space in its place
  
  6.) diskutil resizeVolume disk0s3 296G
  Expand the new boot volume to include the free space I just made
  
  7.) diskutil rename disk0s3 Macintosh\ HD
  Renamed new boot volume to match old one
  
  8.) diskutil eraseVolume "Free Space" Recovery\ HD disk0s5
  Delete faulty recovery partition
  
  9.) diskutil resizeVolume disk0s3 296.6G
  Expand new boot volume to include rest of free space


Since performing this half of my two-part project the partition scheme now looks like this:


  Partition 1, 210 MB: EFI
  Partition 2, 22 GB: Storage 1
  Partition 3, 297 GB: Macintosh HD  


Hopefully this will be helpful for anyone else who needs to move their macOS installation to another partition without losing any data, or anyone who needs to make a bootable backup of their system before performing a potentially dangerous operation.

Now that this part is done, I just need to figure out how to turn "Storage 1" into a bootable installer partition.