I think the more general-purpose suggestion of a Raspberry Pi is going against the desire for this to be simple.
The OP wants an NAS, which isn't unreasonable, as they can simplify the data storage and backup aspects of the process. It would help to have more information such as the type of networking available (wired vs wireless, for instance), the amount of data to be stored, and the need (or not) for other data needs like video and non-media data storage.
Synology is a good option in general. Asustor might have the better hardware, and QNAP makes a wide range of products from small and simple to ones that scale significantly. Terramaster has some good offerings, but I think their hardware and software is not quite as solid as the other. The relative newcomer is UGREEN, and I don't know much about their products, although they seem to be priced reasonably and have generated some buzz recently.
There are several choices here, and with more information on the use case we might be able to pare down the recommendations further.
Michael