Just trying to get myself familiarized with Rhino Mac and Magic Mouse, here are a few suggestions for a better navigation experience in the Mac version.
Zoom in Rhino - feature to turn on/off smooth scrolling: with the Mac-native smooth scrolling on, the delicate zoom control is extremely difficult. (Sometimes, scrolling doesn’t work at all in both Rhino 7 Beta and 6 - strange… ).
Zoom in Grasshopper - zoom with scrolling will be very helpful (with smooth scrolling feature as well).
Three fingers tab for a middle button click - for beloved pop-up menu (on both Grasshopper and Rhino).
Try this for the Zooming speed with your Magic Mouse. Open Preferences (Cmd+,) in Rhino and go to View. Set the Zoom scale factor to .97 or .98, stay between .90 (the default) and .99 in my opinion.
For Grasshopper zooming, hold Alt/Option while scrolling with one finger. The circular marking menu in GH can also be opened with Option+Spacebar which may be helpful as well if you use MMB there.
As for MMB in Rhino with a Magic Mouse, I think this is a limitation of the MM and macOS as far as I can see. I don’t use one of these myself but it looks like there used to be a preferences app you could use to enable a MMB gesture. This may not work with Catalina and I haven’t hunted more.