If you cut power to your Hue bulbs, using the Lutron switch, you will not be able to manipulate them in the the Hue app, or Homekit, anymore.

Take a look at the Aqara hub and motion sensor. You can create automations on thier hub that can send a push notification if their motion sensor is has not sensed motion for a certain amount of time.

I have not used this feature myself, so I don’t know how well it works, but the functionality is there.

r/Aqara is the subreddit for Aqara. You can buy a hub and a motion sensor on Amazon.

I think it would be more economical to do Homekit/Matter. I would not spend the money on Savant and hope the new home owners like it enough to justify a price increase.