I see you're using FMC in front of Bonn switch and server, the most advantageous placement would be after, just before dac. Servers create noise, usb coming out of motherboard is not optimal. Have you ever tried this positioning of FMC? I've always found network upgrades most effective at tail end of network.
You have mentioned this before but I guess I don't understand. My input choices between my server and DAC (both Mojo Audio) are USB, BNC, or AES/EBU, so I do not understand how I would use either FMC or Ethernet for that connection without some sort of converter. The USB output from the server and USB input into the DAC are both well-regarded, optimized for audio, JCAT boards so USB is the connection recommended by Mojo Audio - the server/DAC manufacturer.
I do have the option to put another type of Roon endpoint (Metrum Ambre) that I own between the server and DAC. The connection would be from my server's Ethernet output into the Ambre. I used the Ambre between my Antipodes DX server and Metrum Pavane/Adagio DACs so the DX would not be required to function as both the Roon core and Roon endpoint, and also because the Ambre outputs I2S, which was compatible with the Metrum DACs. Is this the type of connection that you are talking about? If I insert the Ambre between my server and DAC, then I will need to use coax or AES/EBU to connect from the Ambre to the Mojo Deja Vu DAC (since the Deja Vu does not support an I2S input).
For now, the FMC I use is for the 45-foot run from my router to the server. Although I haven't heard a significant difference between CAT8 and FMC for that 45 feet, the trade-offs are that FMC will not pick up RFI/EMI while using the CAT8 allows me to do away with two converters and their associated power supplies. I do not have a clear favorite but I will first get the ENO set up and then try that comparison between FMC and CAT8 again to try and hear a difference.