Point of diminishing return for a DAC

I would strongly consider an R2R DAC. You won't have to spend anymore than specified and there are many out there.

IMO: The point of diminishing returns depends on how much money you have.

Get the MacMini out of the picture by buying something relatively cheap like the following.

https://www.audiogon.com/listings/lis9j9ee-sonore-ultrarendu-network-streamer-ifi-ipower-power-supply-roon-ready-21642-music-servers-docks

It is ROON READY. It is an Ethernet-2-USB converter. Some call it a Digital-2-Digital converter. I use the lower model, the microRendu, and it isolates the computer from my DAC. A used microRendu can be had for $400-$500.  

Computer Audiophile just named the Rendu products as the digital product of the decade. 

$2k for the Matrix XSabre Pro MQA DAC. Best measured performance on Audio Science Review out of 150 tested dacs.

Get an Audio-gd R8 DAC (ladder DAC) and a ultrarendu streamer/bridge and you will be MANY steps above now and VERY close to a multi thousand dollars DACs.