Similiar to ZBXNEXT-7078 an official Zabbix template for OPNsense should be created.
As OPNsense is a fork from pfSense, it should be possible to reuse much of the work already done in https://git.zabbix.com/projects/ZBX/repos/zabbix/browse/templates/app/pfsense_snmp

Since pfSense has gone more of a commercial route, with new features only being added for paying customers, quite different from the approach Zabbix and OPNsense are following, same software for everyone, paying only for support, which was the same with pfSense in the past ;-( many pfSense users switch over to OPNsense. https://opnsense.org/