Your installation of mono is broken. The"mono" found by "which" should be a an executable command/script/program and not a dynamic library (.dylib).
I would delete that entire Mono.framework you installed and then get a new installer.
Download the stable channel latest build, which will end up with an installer .pkg file.
from here:
https://www.mono-project.com/download/stable/
Click the macOS tab (if not auto detected) and then click on the "Download Mono 6.12.0 (Stable channel)" button (on the right side).
Once downloaded, manually drag the mono pkg file to your Desktop and run the installer by double clicking.