The only way to get calendar files (.ics) from iCal calendar backups (.icbu) is to recover the back in Apple Calendar (iCal) and save the calendar data in ICS format. You can always check Apple Store for new apps, maybe some new appeared that can help you with this and automate this procedure.
Convert Apple iCal backup to iCalendar calendar file format (Microsoft Outlook, Apple iCal).
We found one software available in the database for this conversion.
Apple macOS / Mac OS X software - convert icbu to ics on OS X
Apple's personal calendar application for Mac OS X
Convert from icbu file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.