Yes, there is. You can use the script calcardbackup to create *.ics and *.vcf files from the calendar and contacts data in the database. Then you can import those backed up files to a new Nextcloud instance or import them to a client and let them sync the data to a new Nextcloud instance. Though calcardbackup is written for bash, it should also work with openBSDs ksh (from what I read). Otherwise you need to install bash which is also available for openBSD.
IMPORTANT: until release of version 0.8.0 you need to use the testing branch from calcardbackup (version 0.7.2-25 (22.10.2018) or higher)!
create a dummy Nextcloud directory structure for Nextclouds config.php: mkdir -p /usr/local/bin/nextcloud-dummy/config
create a file called config.php in that folder and add the dbtype: echo "'dbtype' => 'sqlite3'," > /usr/local/bin/nextcloud-dummy/config/config.php
restore the SQLite3 database from your backup in the dummy Nextcloud directory created in step 1: sqlite3 "/usr/local/bin/nextcloud-dummy/owncloud.db" < "/path/to/nextcloud-sqlite3.bak"