You use CoW on filesystem level and take snapshot every 5 mins. Your concerns about write coming from above is gone.
The point about being able to use different media like bluray drives is a valid point but since xz doesn't do any correction it doesn't really matter, it has to be done out-of-band anyway.
Right, but my concerns about an overly complex system coming in as a stand-in for a simple one are not.
The simple cost effective thing is not to engineer a complex redundancy system above and below to try to adhere to some misguided "separation of concerns", its to use the simplest, most effective solution which presents itself.
When you try to separate things which should not be separated in a software (or other) system, you get high coupling, low cohesion. Not everything should be attempted to be "de-coupled".
The point about being able to use different media like bluray drives is a valid point but since xz doesn't do any correction it doesn't really matter, it has to be done out-of-band anyway.