Consider this separately:
- What is the granularity of the events published in the sdm feed
As you’ve pointed out, what is happening in google home app doesn’t matter because the feed is different. Stop talking about google home What matters it the events published by the SDM feed. So what are you seeing? If you are seeing the feed actually publish something useful at a granularity that you can’t see today via home assistant, show me an example of the messages received and the deltas in timestamps.
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How to get notified via event payload
I discussed the caveats for this, and why even if #1 is useful why its a challenge for existing events. But lets stop talking about this for now and focus on #1. -
How to store the state?
As i mentioned before a few times, there will be no new state added, and you should use template sensor to store whatever you want from the event payload. There is nothing else to discuss about this that hasn’t already been said a couple times already.