that's just a false sense of security. A machine should be treated as a whole. Separate components only matter economically, not for data security.

My macbook updates my documents etc to my NAS, and the NAS syncs to crucial folders to the cloud (not everything for cost reasons)

You should always assume your device can be stolen, you can spill fluids on it (or a guest), a cat can pee on it, an electric shock may kill it... No matter how great care you take of it, shit happens, data needs to be safe.

The SSD dying standalone vs the system dying doesn't change at all for data security. It's not a different risk factor