Remember the [integration.mqtt] section is only for the “integration” events. You could leave this pointed at your third party broker with tcp://broker-ip:1883 even if TLS is set up on your Chirpstack broker
But if you wanted to implement TLS on your Chirpstack MQTT broker you need to follow the guide. But yes that includes mounting the certificates to chirpstack, but adding them to the [gateway] and [integration.mqtt.client] sections, not the [integration.mqtt] section. The [integration.mqtt.client] section is what actually allows you to click that button in the web UI, and through clicking the button in the web UI, you can get the certificates to put in your chirpstack-gateway-bridge.toml.