I know you mentioned standalone, but have you considered the modular approach where the files are kept on a separate computer(somewhere else in the house), something that could be much less than a mac mini in price (even build one from old PC parts lying around, or a refurbed (aka off lease) box, which are available for $150-200, and then have a small "bridge" device like a Raspberry Pi, as mentioned above, (or my current favorite Toy, a Nano Pi Neo 2 from Friendly Arm) driving your DAC?
There is a software client for MPD that has me very interested, that to me looks like it could be a no cost alternative to Roon in a lot of ways (since it pulls in metadata from various sources like Roon), called RompR.