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Move the motors by hand while LinuxCNC is running but not enabled, do the numbers on screen (Gremlin) change?

if you have two (or more ) Tangnano, you can try it, you need the 'dev' branch and
2 free pins on each tangnano (RX / TX).
For short distance, you connect this pins direct (RX<->TX) or using RS485/RS422 converter for long distance.

Rod, he has huge excursions on both sides of the graph, those are not shown unless using --show, and those are the ones causing latency errors and link drop.

Look forward to seeing how that goes, Tang Nano's are about 30 bucks AUD, two of them are still cheaper than any FPGA with a similar total pincount. I'm being selfish but that also means I stick with the same FPGA hardware for every machine I can possibly imagine making. 

Let me know if I can help with testing, I'm currently retrofitting my three machines to closed loop to the controller, so there's room for some hardware experimentation. 

A broader opportunity could be to try and get involved in plant automation. This will get you using a lot of fieldbus technology like ethercat which is transferrable to CNC machines which are only a very small subset of this automation industry. 

Removing the Wifi board is probably good for latency.
The DKMS driver can usually be installed with 

sudo apt install r8168-dkms
but you do need to have non-free in the apt sources. This is included in the Linuxcnc ISO

Seems the card uses a R8168 interface. The R8168 requires a DKMS driver to get acceptable RT  performance. Not sure how that goes with a Pi. I would not use it for ethercat but would be fine for general network use.