Even prior to docker-slim there were tools like Quay.io that "did the right thing" by squashing images to just the contents of the final image layer.
The best thing you can do is use minimal images and multi-stage builds. This should help you immensely to reduce your attack vector and do standard software bill of materials, too.
The quay.io squashing optimization is a lot safer though, right, as it doesn't remove anything that should be visible to the container?
I agree that the multi-stage builds are the best option, but it can be hard to know if you've included everything that is required or if you've accidentally excluded something that is important in rare cases.
The best thing you can do is use minimal images and multi-stage builds. This should help you immensely to reduce your attack vector and do standard software bill of materials, too.