@chunhungyeh This worked!! Thank you so much!
For future replication/to summarize, I downloaded the QT installer, uninstalled the current qt version with
brew uninstall --ignore-dependencies qt
if it was installed by homebrew, or by running the “maintenance tool” in the qt install directory if it was manually installed. I then used the downloaded QT installer to set up specifically QT version 5.12.1 to the /Users/username directory, and then ran
export PATH=/Users/username/Qt/5.12.1/clang_64/bin:$PATH
brew reinstall mrtrix3
Which allowed me to use MRView as needed/with tools, and I was able to open my .mif images.