Most SSDs have a lot of redundancy in the form of multiple flash chips, it's just not exposed to you, the user, by the controller chipset.
There are also PCI-E x16 with multiple NVMe slots. There's nothing stopping some company from making something similar for Thunderbolt.
And I guess the cost is too high and the connectors would be too bulky, but it would be nice to have something like a SODIMM for flash that separates the memory from the controller.
Are we really comparing the "redundancy" of an SSD internal workings to a NAS? You're reaching really far for that. Whatever "redundancy" an SSD has is whatever it needs just to present a single instance of that data to you. Sounds pretty damn ineffecient, but if that's what it takes to reliably get 1200MB/s speeds I'm okay with it for situations where I need that.
The redundancy a NAS can provide means losing an entire drive (or more depending on config) in the unit without data loss. Even with that failed drive, the data is still accessible. The failed thing can then even be replaced and the system can be brought back to the normal full redundant state. Your "redundancy" of an SSD doesn't allow for that to happen.
There are also PCI-E x16 with multiple NVMe slots. There's nothing stopping some company from making something similar for Thunderbolt.
And I guess the cost is too high and the connectors would be too bulky, but it would be nice to have something like a SODIMM for flash that separates the memory from the controller.