That setup is not separating Core and Output. The Zenith is being used as both Core and Output. In order to separate Core from Output you either need a Roon Ready device or a network endpoint running Roon Bridge. If you used the NUC then the signal path would be as follows (assuming music files stored on USB or a further internal SSD on the NUC):
NUC (Core) >>> Ethernet >>> Router/Switch >>> Ethernet >>> Endpoint Device (Output) >>> USB >>> Phoenix >>> USB >>> DAC.
I don’t know if there will be any SQ difference, haven’t heard the setups to compare. The NUC should handle up to DSD 512 , but probably not noiselessly. The idea is to put the NUC in another room near your Router and just use the low footprint network endpoint device near your system.
I use a Sonore microRendu as a network endpoint device, it has Roon Ready and HQP NAA software.
You will notice that with this topology the NUC is in another room. So why use a NUC ? You can probably build a server in a larger case for less cost but the NUC can run ROCK. I use a server running Linux because it also runs HQP.