Point of diminishing returns for a Network Streamers?

I have a network connection at my system. So the streamer would not be connected to the computer. The mac would, for now, be the Roon Core only and be discoverable on my netwrok. 

If you don't need volume control and you use Roon as your interface the Pi4 running Roopie would be fine. The iFi zen is a good choice if you aren't inclined to put a raspberry pi togethe and load an OS. 

+1 @soix regarding the excellent performance and value of the ifi zen stream... tremendous value, just a terrific sounding product at a very very modest price, does exactly what it is designed to do, no frills but simply superb in its targeted functionality imo

agree also with other comment that a good linear power supply into the zs is essential for best performance

Your Luxman integrated deserves a quality digital front end, if your DAC is on par with the Luxman, purchase as follows:

ADOT MCO1 fiber optic kit

LHY 25A LPS with dual 5v outputs

Uptone Audio EtherREGEN switch

Audioquest Forest ethernet cables

Lumin U2 

The Lumin alone is a significant upgrade over the iFi Zen, but coupled with this chain, the sonic gains can be huge.

How do I know this? Because every single step I mention above I have done in both my home and office systems, with fantastic results.

If you are using usb to any dac, you will have inferior sq.

what is a streamer? A computer running a proprietary OS usually running on hardware with gimmick technology to promote a higher price.

Keep any server out of your audio room and use a better interface to the dac than usb. 
Why would anybody think a pi is a better server than say a Mac mini? You think it’s because of Linux? Think again. Every 3rd party streamer uses some version of linux, some with tainted OS’s because the vendor thinks they have a tweak that performs miracles, I would never run a tainted linux because if anything happens to the streamer or the company goes out of business, who will support this linux version? Good luck