The How-To Migrating from User to Service install on Windows (latest recipe is v3) covers doing that.
It’s much simpler if you’re starting from scratch, but presumably you have backups you want to move.
Additionally, your steps would still leave Duplicati’s TrayIcon wrongly thinking it should start its server. Connecting to that would put you at port 8300, not 8200 which service likely got (if it got there first…).
There might be other issues I forgot, so follow the How-To. When at right server, configure snapshots.
About → System info UserName and StartedBy will be clues to which server you got (if it’s not clear).
Windows service by default installs as SYSTEM so has plenty (maybe too much) Administrator power without having to jump through hoops (such as UAC) that ordinary Administrator group would require.
Duplicati GUI is just a web page, so user doesn’t matter any. You would browse directly to the service. Getting TrayIcon client going is kind of complicated, if you have a GUI lock. If you want to do that, ask.
Basically, it needs to get into the server that the service started, and can hit access permission issues.
EDIT:
A full set of service, server, and TrayIcon (with desired --no-hosted-server) would look like image at
[SOLVED] Is it ok that I see 5 processes of Duplicati in Windows Task manager?