I’ve been thinking about the suspend / resume as I’ve been working on a separate implementation of chromecasts / google homes as smartthings devices
I don’t think there’s any native capability of the chromecasts to resume / restore an app, so potential solutions are:
delay the notification until the current app is idle (that may be okay for some kinds of notifications, but not so much for others)
If the current app is an app that can play an audio file (DefaultMediaReceiver for sure, but probably not many others), insert the notification at the beginning of the queue
Look into saving current application state and restoring it afterwards (that might be possible for some where the protocol is known, like Youtube).