The question is: To use Airplay or not to use Airplay.
Before buying anything, try your last idea: move the MacMini and see how you like the sound quality with it serving the DacMagic directly. My MacMini is dead quiet; the fan never comes on and I can't hear the hard drive spin up.
Then I would try the free 2week Roon trial. It runs on the Mini, uses your iTunes library without modifying it in any way, and - if you decide to keep Roon - will provide the core technology for you to add an inexpensive end point where your Apple TV is now.
Besides being a terrific music library interface, Roon has RAAT technology, a new and fast growing method of transporting digital audio from your server (in this case the MacMini) and the rest of the house when you move the Mini back to where it is now. This includes both standard resoluttion and hi-res audio. Remote control is available using later generations of Apple and Android devices running the Roon app. (Free w/ Roon)
At that point you might look at replacing the DacMagic with a device that is Roon Ready and/or offers other features like DSD processing.
Your system as it stands now is pretty good, IMO. ( And elegantly simple using the Apple remote app.) There's plenty of iTunes (music player) critics out there but I ain't one. Also, Roon will run your existing config. It is compatible with Airplay devices.
