Currently we are providing tar.gz archives for both sfizz library and LV2 plugin.
On Windows we have an InnoSetup script to build an installer setup, via Travis-CI, for the LV2 plugin; it would be good to have one also on macOS.

On macOS does exists a procedure to create .pkg installer package files using pkgbuild, productbuild and pkgutil tools; there are also some frontend utilities to simplify the process. I started using macos-installer-builder (here an article from the author) but it seems there is also another interesting one called Packages.
The purpose is to create a final bash script to use in Travis-CI, so no GUI based builders.

As a side note, similar on InnoSetup on Windows, macOS packages can be publicly signed and an Apple Developer account is required to do it. On latest macOS versions signing is mandatory (see "notarization").

Another additional option could be to distribute the resulting package using a dmg image, usually built using hdiutil. This could be also achieved by using create-dmg, "a shell script to build fancy DMGs", so it can be executed inside the CI build.