Digital source upgrade suggestion?

AirPlay isn't ideal because it's limited to 44.1 kHz / 16 bit. If you want to use listen to high-res material, you should use UPnP instead.

And you're right - if the Bricasti has UPnP capability, then it can already serve as a media renderer for playback. Buying an external streamer / server isn't necessary.

To stream to the DAC, you need to do a few things. The first is to set up a UPnP server. This can be done on your MacBook or directly on the NAS itself, if you go that direction.

I use a free and open source app called Kodi to enable UPnP on my Mac mini. Once installed, you need to add your iTunes Library as a music source. Then you need to enable UPnP on Kodi, including the options to share your music library, look for remote players (will need to show advanced options by click on the gear icon in the lower left of the navbar until it says 'Advanced'), and allow remote control via UPnP.

If you'd rather use a NAS such as Synology, there are packages available to enable UPnP functionality.

You can control playback and stream to your Bricasti DAC by using a UPnP app. For iOS devices, I suggest mconnect Player HD. The Bricasti should appear as an option when you tap the 'Play to' button the upper left. If you're on Android, I suggest BubbleUPnP. The Bricasti should show up as an option if you tap on the cast icon on the lower left.

Alternatively, you can also use the Kodi app interface directly by right clicking on a song and choosing "Play using". This is a bit clunky though.

I also forgot to mention that mconnect and BubbleUPnP feature integration with Tidal / Qobuz so you can use them to stream from those services to any UPnP client.

I suggest you look at a dedicated music server like the i5 from Green Computer.  You can attach your 4tb drive and connect to the Bricasti and run Roon from the i5.

My setup is pretty much the same.  The beauty of it (WiFi) is that nothing on computer side matters (Computer speed, power supply, amount of memory, playback program etc), since there is no direct connection and no real time clock .  Airport receives bit perfect data in packets and converts it to music by adding timing.  AE is  pretty jittery on analog out but Toslink output is not that bad producing about 280ps of jitter.  AFAIK about 50ns becomes audible so some reduction is in order.  I use jitter suppressing DAC.  

Itunes is different story.   User interface is clumsy, so I created "smart playlists" for each gender  (Gender is ......).  After that each gender smart playlist shows all CDs on one screen as icons.  Unfortunately it is still not showing full screen because, for some reason, they decided to use almost half of the screen for the banner.  I had enough of this stupidity and bought inexpensive Jriver Media 29 (now Media 30).  Once you figure out proper settings it is pleasure to use.  Icons are displayed on whole screen, making it easy to browse thru records. My music is on external SSD connected to Mac Mini..

@yage Thanks for these suggestions, very insightful