Improvements

The package must be arch ALL, not arch any. It only contains a bash script. You should also remove ${shlibs:Depends} from the dependency.

You can remove the file debian/betterlockscreen-docs.docs: there is no binary package named betterlockscreen-docs.

The debian/watch doesn't work (see the issue on i3lock-color for information).

The debian/betterlockscreen.service is a copy of betterlockscreen@.service: remove the debian file and install the upstream file by adding in the debian/betterlockscreen.install

betterlockscreen@.service lib/systemd/system

And you can remove the related override in debian/rules