未測試, 可能還有沒考慮到的, 算是給個魚桿了 
 
input_number:
  light_blahblah_brightness: {initial: 0, min: 0, max: 255, step: 1}
sensor:
  - platform: template
    sensors:
      light_blahblah_brightness:
        value_template: '{{ states("input_number.light_blahblah_brightness") | int }}'
automation:
  - alias: when_input_number_changed # input_number 數值變化時 反應至實際亮度
    trigger:
      - platform: state
        entity_id: input_number.light_blahblah_brightness
    action:
      - service: light.turn_on
        entity_id: light.blahblah
        data_template:
          brightness: '{{ states("input_number.light_blahblah_brightness") | int }}'
  - alias: when_light_brightness_changed # HA 啟動時 以及 亮度 sensor 變化時 反應數值至 input_number
    trigger:
      - platform: state
        entity_id: sensor.light_blahblah_brightness
      - event: start
        platform: homeassistant
    action:
      - service: input_number.set_value
        entity_id: input_number.light_blahblah_brightness
        data_template:
          value: '{{ states("sensor.light_blahblah_brightness") | int }}'
 
  
 
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