Is it possible to change the default duration for a new event in Calendar from 60 min to 30 min
Hi and welcome to Ask Different. 

Yep you can change the Mac's Calendar default duration for a new event using a terminal command. 

Go to Applications>Utilities>Terminal and enter the following command in terminal: 

defaults write com.apple.iCal 'Default duration in minutes for new event' 30

The default duration should then be 30 minutes. You can change it to another value by changing the number 30 to whatever you wish.

If you wish to revert back the default 60 minutes, you can enter the following command in terminal: 

defaults delete com.apple.iCal 'Default duration in minutes for new event'

This works on macOS Mojave 10.14.6 and suspect it does for macOS Catalina too.

Hope this helps!