MQTT get state from template

Hi Community,

I want to create a MQTT cover - a garage door. I have installed the sonof RF Bridge and created the open and close commands for the cover. It works. The state is assumed.

Today I have installed an ESPhome binary sensor (magnet reed) to check the real state of the garage. I have tried to use the state directly in the MQTT config. But it doesn’t work:

 - name: "Garage"
    unique_id: garage
    device_class: garage
    command_topic: cmnd/tasmota_012A6A/Backlog
    payload_open: rfraw ...; rfraw 0
    payload_close: rfraw .,,; rfraw 0
    payload_stop: rfraw ...; rfraw 0
    state_topic: "{% if  is_state('binary_sensor.sensor_garage', 'off') -%} closed {%- else -%} open  {%- endif %}"

I know the template is no topic, however I wanted to return closed or open ( The MQTT topic subscribed to receive cover state messages. State topic can only read (open , opening , closed , closing or stopped ) state.)

Does this even work or do I have to create a separate cover-entity?

Best regards,
Konstantin