Setting up a VPN tunnel using Tailscale might work, if you can install that on the Synology . TS network on the NAS to TS Network on, say your phone/tablet/pc, then Chromecast to your Wiim.
Having said that, me and the wife can use Plexamp or Symfonium to stream music to the car stereo over our phones 4G/5G from our NAS, so that might be enough for you?