With the upcoming release of IPFire CU 204 the Knot DNS Resolver (kresd) metrics will be exposed by the Zabbix Agent addon for monitoring by Zabbix.
To fully utilize those metrics, I have created a new Zabbix template: GitHub - RobinR1/zbx-template-kresd: Knot Resolver statistics by Zabbix Agent Active · GitHub
To start monitoring your IPFire Knot resolver performance, you only have to import the template and assign it to your IPFire CU 204+ host in Zabbix. No additional configuration is required on the IPFire side (except of-course for having the zabbix_agentd addon installed)