It’s mostly for internal stuff with a NAS. Uploading and downloading files off a NAS or streaming 4K content can all benefit from 10G

just get a cheap nas eg from synology

Cheap and Synology are mutually exclusive. But I agree with the rest of what you said.

It’s not a good idea due to the load on the router and a proper NAS would be better

Because SDN setups are significantly better than Mesh