What is reading it in your case, or is your question hypothetical?
Last changed is the previous time the state changed, not the time there was last a state written to the DB (it doesn’t have anything to do with the DB). There’s a bit more to it, like the difference between this and last updated and how an HA restart affects this. I’m not sure what you mean with parallel.
Pedantic note: last_changed and such are technically state properties and not attributes.
Many people do similar things, but YMMV. It depends on many things and probably some trial and error. I personally have created a reverse situation: I use the state of a specific light (the main living room lamp) to trigger many automations and to use it as a proxy for presence. It’s very simple and for years hasn’t had issues with edge cases. Granted, it’s just me and my wife. Things will be interesting with children.