I would consider adding a WiiM Pro ($169, link) if the streaming interface is the real pain point. You already have a NAS and a good sounding DAC in the Marantz. The WiiM Pro would replace the streaming function and can be connected to the coax digital input on the Marantz with the appropriate cable.
The WiiM Home app supports Amazon, Spotify, TIDAL and internet radio via TuneIn / vTuner / Radio Paradise / etc. However, it doesn’t support directly connecting to a network share at the moment. That means you’d need to enable the UPnP server on your NAS so the app can browse and playback your local music collection.
The best part is that you can buy the WiiM from Amazon direct and return it if you don’t like it.