Help me wrap my head around Google Calendar

Configuration
3

I haven’t worked with the built-in offset settings (!!), but I wanted to have an event that alerts me 15 min before an event ends (so that I’m reminded in time to pick up my son from school). I didn’t have any luck getting this to work as a template trigger, so I created a sensor (with the same template rules), then created an automation that watches the state of that sensor. I ended up creating several sensors that all worked about the same way (various weekly kid school-things), so I set up a sensor for each, then one automation to remind me.

The various Google calendar entities are searches on one calendar.

Here’s my code:

calendar:

- cal_id: [email protected]
  entities:
  - device_id: nathan_tech_shop
    name: Nathan Tech Shop
    search: Nate to tech shop
    track: true
  - device_id: nora_mathcounts
    name: Nora MathCounts
    search: Nora MathCounts
    track: true
  - device_id: nora_chessclub
    name: Nora Chess Club
    search: Nora chess club
    track: true

sensor(s) - repeated for each calendar entity. 900sec = 15min. Note that I have a sensor.date__time that keeps the current time. I use that in the sensor so that it updates the value whenever the time changes (every minute).

      nate_tech_shop_end:
        friendly_name: Tech Shop ends in 15 minutes
        entity_id: sensor.date__time
        value_template: >
          {% if states.calendar.nathan_tech_shop %}
          {% if is_state("calendar.nathan_tech_shop", "on") and as_timestamp(states.calendar.nathan_tech_shop.attributes.end_time) - as_timestamp(now()) < 900  %}on{% else %}off{% endif %}
          {% else %}off{% endif %}

automation (I used meaningful names in the sensors so that I could just reuse them as part of the notification):

- alias: Time to pick up a kid notification
  trigger: 
    - platform: state
      entity_id: 
        - sensor.nate_tech_shop_end
        - sensor.nora_mathcounts_end
        - sensor.nora_chess_club_end
      state: 'on'
  action: 
    - service: shell_command.speak
      data_template:
        text: >
          {%- for state in states.sensor -%}
           {% if state.entity_id == trigger.entity_id -%}
             {{ state.attributes.friendly_name | urlencode }}
           {%- endif %}
          {%- endfor -%}
    - service: notify.scott_notifier
      data_template: 
        title: "Time to go!"
        message: >
          {%- for state in states.sensor -%}
           {% if state.entity_id == trigger.entity_id -%}
             {{ state.attributes.friendly_name }}
           {%- endif %}
          {%- endfor -%}