Above was an invent-it-yourself install to bypass AUR. Might be easier to work with AUR.
Clues currently on the front page for Canary, and same clues probably apply to the Beta.
https://aur.archlinux.org/packages/duplicati-canary-bin
More clues:
https://aur.archlinux.org/packages/duplicati-beta-bin
I suspect several of the why-did-my-AUR-update-break-Duplicati have to do with unit files.
I don’t run Linux much, but have had to learn some systemd. Are you good with systemd?
Modify systemd unit file without altering upstream unit file
Modify systemd Config Without Touching Upstream Unit File
How to Change the Default Editor for Editing Unit Files in Linux
might be the thing to do to customize the unit file with lower chance of overwrite on update.