That is awesome! One issue I noticed using an espHome slider is that it would send a bunch of PUT requests during user input. If this becomes an issue, I think you can debounce the Automation by adding a 2s delay to your automation:

    actions:
      - action: rest_command.esp210_output
        data: '{"brightness": {{ states("input_number.esp210_brightness") | int }}}'
      - action: homeassistant.update_entity
        data:
          entity_id:
            - sensor.esp210_output_sensor
      - delay:
           seconds: 2

Oh, I haven't noticed any issues, but it's a good idea