Hm, the issue with libusb on macOS seems to be gone when I use LimeSuite built from source whereas the version in the Pothos Homebrew tap (brew install pothosware/pothos/limesuite) produces the libusb errors.
I also noticed that installing Gqrx via Homebrew-Cask (brew install gqrx) creates symlinks for SoapySDRUtil pointing into the Gqrx application:
/usr/local/bin/SoapySDRUtil -> /Applications/Gqrx.app/Contents/MacOS/SoapySDRUtil
This means Gqrx ships its own SoapySDRUtil. This version did not include the lime driver (SoapySDRUtil --check=lime returned Checking driver 'lime'... MISSING!), so Gqrx also did not provide the LimeSDR in the device dropdown. I then reinstalled SoapySDR via Homebrew (brew install pothosware/pothos/soapysdr && brew link --overwrite soapysdr to overwrite the exisiting symlink) which at least made SoapySDR find the LimeSDR Mini.
Unfortunately Gqrx still does not work with the LimeSDR Mini: When I start it from the command line via /Applications/Gqrx.app/Contents/MacOS/gqrx I see a line with following error:
FATAL: SoapySDR::Device::make() no match
This seems to be a bug in Gqrx or SoapySDR, as I get the same error on my Gentoo Linux installation. Also see Gqrx for limeSDR on RPI3 where the maintainer of Gqrx addresses this error.
Edit: There is this Gqrx GitHub issue which might help fixing the SoapySDR shipped with it: https://github.com/csete/gqrx/issues/556