I’ve come up with an easy (5 minute) solution to start a print at a particular time of day (or night) using Automator and Calendar both in MacOS. And once you have created the little app to start the print (using Automator), and the calendar event, you can just change the day/time of on the calendar event. Simple 
Create a calendar app which clicks the Send button in the slicer
- open Automator (Applications folder)
- select Calendar Alarm
- type Watch in search field (top left)
- drag Watch Me Do to the right panel
- get ready to print in your slicer e.g. ‘Send print job to’ window
- in Automator with Watch Me Do, click Record (top right)
- click the Send button
- stop Automator recording - click stop button on small popup window, probably to the left
- click Run (top right) to check it does click the send button (you need send window open)
- File → Export and name the app e.g. start print.app
Allow the app to control your computer
- Apple Menu → System settings → Privacy & Security → Accessibility
- press + and add your app
Add event to your calendar
- add new event on the day you wish to start your print
- set the time
- click Alert and set action to Open File
- click Calendar and select your app e.g. start print.app
- set time to ‘At time of event’
- Close
Get ready to Send print job in your slicer
Notes:
Ensure that your display does not turn off otherwise the screen event you want (i.e. clicking on a button) will not happen.
- System Settings → Lock Screen. Turn display off when inactive - give enough time for the print to start.
- you can always turn the display brightness down
Accessibility error message
- you may get this if you have edited the app in the Automator. Follow the steps above in ‘Allow the app to control your computer’, deleting the old app first, even though the name is the same.