One disadvantage of a Startup shortcut with “Run this program as an administrator” is that at login time it will give you a UAC prompt every time (if you have UAC turned on at that level).
The Run as a service, and task manager methods don’t prompt UAC.
Also, note, if you “run as a service” you will hit the problem that after you’ve Installed Windows 10 Fall Creators Update - Now Everything is Missing In Duplicati. You can adapt these instructions to change the location of the duplicati datafolder to avoid that problem.