Per the documentation there should be an option to allow Secure Folder calendar to sync out to the default calendar. However, I'm unable to duplicate it. The contact information shares fine, but there is no option visible for calendar as seen in my screen shot below.
This is extremely frustrating. The only way I can get my work calendar and personal calendar merge into one display is to have the work calendar shared to an ics file and imported as a 3rd party into my private calendar. This is automated, but only refreshes when my work computer is running Doesn't help much when things are added or removed outside normal hours.