50’ Connection Between Mac and DAC

You can: 1) Connect the mac mini to your wireless  router with an ethernet cable 2) Connect the Raspberry P i to the wireless router with an ethernet cable 3) Connect the Raspberry Pi to the Schiit Bifrost 2 with a USB cable. 4) Install ropieeexl  on the Raspberry PI 5) on the mac mini install Roon server or Audirvana

I tried this.
Waste of time.
Did not work.
It would probably sound like rubbish if it did.
Settled on line out from Schitt DAC and a RF remote to control the computer.
Then I grew up.


For what you gotta spend on a decent balanced cable and interfaces you will get better SQ AND control with a streamer.

Are you currently using the internal DAC in the Airport Express? If so, just stick with your wireless setup and get a decent DAC. You can use the optical out of the Airport Express into a DAC. It will give you the biggest improvement in SQ. 
My setup is similar to your setup with loseless ripped CDs on a Mac Mini. When I compared my wireless setup to a wired setup, it was either hard to tell any difference or the wireless sounded better, depending on the cabling that I used.

OP
SPDIF stands for Sony Phillips Digital Interface.  It is also known as a Coax cable.  It is one of the two digital cables originally intended
to connect two audio components ( usually a CD player to DAC, or a device that contains a DAC, such as an Audio Visual Receiver (AVR).
The other common cable for this is an Optical Cable, synonymously known as TosLink
.  Coax and Toslink both achieved prominence in the nineties and are still used.  They are both meant for short runs.  I don’t know if it’s possible to do what you outline in your OP.
   Computers tend to rely on USB, or Universal Serial Bus, links, that were designed for devices such as printers.  Many computers don’t even have Coax or Optical outputs—check to see if yours does.
   Streaming allowed for greater distances between where the music
is stored (a hard drive somewhere) and where it is played back (audio system).  Streamers are devices that essentially facilitate this.
This can be done wirelessly, or using other digital cables meant for longer runs, such as Ethernet cable, which resembles traditional telephone wire.
  Regardless of cable used, you will still need a Digital Audio Converter, or DAC.  DACs convert all the 1s and Os into actual music, an analog wave form.  As mentioned  earlier, DACs can be part of an Audio Component, such as an AVR.  Computers have internal DACs, so you technically have one, but they are noisy, poor performing devices.
   Which brings me to the point: it sounds like you need a DAC, and as others have pointed out, a streamer will solve your needs better than a long cable run.  Since DACs and streamers frequently are paired together in one component, that is the direction I would go. You can also use the DAC to improve other digital sources that you may have in your audio system.I would think this over, then if you agree, define your budget, and off you go

I agree with ljgerens. If what you have is an Airport Express model A1084 or A1264 with the combination optical and analog output, using a mini-Toslink to Toslink optical cable to connect over WiFi to a DAC is an excellent option. Any DAC with an optical input will work, I've been using an AudioEngine D1 for years to connect to my garage system.