High Sierra, how to set screensaver as desktop background?
The path and functionality have seemed to change. Heres a link and quoted tutorial to do it. Unfortunately, it appears its much more of a hassle in High Sierra+.

From here:


  If you are using High Sierra (or later), the ScreenSaverEngine.app has
  been moved to a different location. Use the code below instead of the
  one above.

/System/Library/CoreServices/ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine -background &amp;

  
  The problem with running just that line of code is that if you close
  the Terminal window — or if your Mac goes to sleep — the screen saver
  closes and your wallpaper goes back to whatever it was by default. To
  handle this, we need to go a bit deeper.
  
  To detect when the Mac sleeps and wakes up, we need a small piece of
  software called “Sleepwatcher.” You can download it here. Just open
  the file and your Mac will extract the downloaded file (sometimes it
  might have to be extracted twice). After extracting, you’ll get a
  “sleepwatcher_2.2” folder. Just move this folder to Desktop and run
  the following lines of code in the Terminal.

sudo mkdir -p /usr/local/sbin /usr/local/share/man/man8

  
  You might need to enter your password after pasting this line. Next,
  run:

sudo cp ~/Desktop/sleepwatcher_2.2/sleepwatcher /usr/local/sbin

  
  Then run:

sudo cp ~/Desktop/sleepwatcher_2.2/sleepwatcher.8 /usr/local/share/man/man8

  
  Awesome! You have successfully installed Sleepwatcher. Now let’s add
  the lines of code needed to make Sleepwatcher run the screen saver
  when your Mac wakes up, and kill the screen saver when your Mac goes
  to sleep.
  
  Sleepwatcher searches for and runs two files, .sleep when the Mac
  sleeps, and .wakeup when the Mac wakes up. We just need to create
  these 2 files in the user’s Home Directory.
  
  In the Terminal, type nano ~/.wakeup then paste the below code.

 #!/bin/bash
osascript -e 'do shell script "/System/Library/Frameworks/ScreenSaver.framework/Resources/ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine -background &amp; EOF"'

  
  Now press Control + X to exit. When it asks if you want to save the
  file, press Y and then press the enter key to confirm the file name.
  This will create the .wakeup file. Now to create the .sleep file.
  
  
  
  Just like above, type nano ~/.sleep and paste the below code.

#!/bin/bash
osascript -e 'do shell script "kill `ps -ax | grep [S]creenSaver | cut -c1-6` EOF"'

  
  Again, press Control + X to exit, Y to save, and then the enter key to
  confirm the file name. Now the .sleep file will be created.
  
  
  
  In Terminal, run the below line of code.

chmod 700 ~/.sleep ~/.wakeup

  
  It changes the permissions for the newly created files so that it can
  be run by Sleepwatcher.
  
  Now that you’ve created the scripts, you just need to add Sleepwatcher
  to launchd so that it can start when the system starts, then
  continue to run in the background. Paste the following code code into
  your Terminal.

cp ~/Desktop/sleepwatcher_2.2/config/de.bernhard-baehr.sleepwatcher-20compatibility-localuser.plist ~/Library/LaunchAgents

  
  This will copy the Sleepwatcher property list file so that it can be
  added to launchd. Now just paste the code below into Terminal to add
  Sleepwatcher to launchd.

launchctl load ~/Library/LaunchAgents/de.bernhard-baehr.sleepwatcher-20compatibility-localuser.plist

  
  Awesome! You can now restart your Mac and the launchd will run the
  Sleepwatcher scripts at the startup. Just put your Mac to sleep and
  wake it up. Then you’ll be welcomed with a beautiful wallpaper.