What you could do is to create a new database, import the old database backup to that database and export the calendars with the help of a dummy config.php and calcardbackup. Have a look at the section does this work with a broken instance in the calcardbackups’ README for a more detailed walkthrough of the procedure.
In my opinion the best way to import the calendar files to the fresh instance is to import the exported *.ics files to a caldav connected client and let that client sync the calendar to nextcloud.