Ok thanks for the info. Is the SFTP destination on the local network or is it remote?
Can you open a browser tab to Duplicati web UI -> About -> Show Log -> Live, and set the dropdown to “Verbose”. Leave this tab visible - don’t close it.
Open another tab and start the Duplicati backup job. When it appears to hang at the end of the job completion, go back to the Live Log tab. Do you see any updates regularly there? Or does it stop for a long time (20+ minutes) with no further updates? Copy/paste the most recent entries here.