Turning your NAS into a relay would allow you to access devices inside your home LAN, from outside your home LAN. This could include accessing devices inside your LAN that don’t have Tailscale installed, such as your smart TV.
Remote access could be from your laptop or mobile phone, which is outside the LAN, and signed into Tailscale. Your laptop or mobile phone would “see” the NAS as a relay, and recognise IP addresses that the relay advertises (IP addresses for any device on your home LAN, such as your smart TV). Only devices with Tailscale installed can “see” non-Tailscale devices that are located in a LAN advertised via the relay feature.
But even if you do all the above, this lets you go from Tailscale device → non-Tailscale device. E.g you can access your smart TV from your laptop. But it sounds like you want to go from non-Tailscale device to Tailscale device, e.g. smart TV to (remote?) NAS. However that’s not going to work because you can’t install Tailscale on your TV and only devices with Tailscale installed can see other devices in the Tailscale (virtual) network.
If you want a friend or family member’s remote laptop to be able to see your NAS, all you need to do is sign into Tailscale on their device, and they will see your NAS straight away. No relay feature is needed as Tailscale can see all other devices signed into Tailscale. If you wanted a friend or family member’s remote smart TV to access your NAS, then you have the same problem as above: only devices with Tailscale installed can see other Tailscale devices, or other non-Tailscale devices advertised by the relay feature. The “source” device has to have Tailscale installed to see other Tailscale devices and relays.