Automatically open Finder when disk image is mounted
As a test, I created a 100 MB APFS disk image and saved it to the default location, in my Documents folder. I then unmounted the disk image, as it needed to be unmounted to test what I did next.

Next, in Automator, I created a new Folder Action, while setting Folder Action receives files and folders to Volumes. I did this by clicking Choose folder > Other… and then pressed ⇧⌘G to bring up the Go to the folder: sheet. I then typed in /Volumes and clicked the Go button, followed by the Choose button.

Next I added an Open Finder Items action and saved the Folder Action as e.g. Open Mounted Disk

From Finder, I then doubled-clicked the APSF.dmg file and when it was mounted, it opened a new Finder window to the root of the mounted disk image.




Note that when the disk image opened, I was prompted with a message "FolderActionsDispatcher" would like to access files in your Documents folder, to which I clicked the OK button. This created an entry in System Preferences > Security &amp; Privacy > Privacy > Files and Folders as FolderActionsDispatcher with a [√] Documents Folder checkbox.
The e.g. Open Mounted Disk.workflow was by default saved to: 

/Users/$USER/Library/Workflows/Applications/Folder Actions/