Linux Mint - PlasmaC upgrade confusion

I'm trying to upgrade PlasmaC and getting rather confused. So far I've updated LinuxCNC (sudo apt update / sudo apt dist-upgrade). I'm running Linux Mint 19.1, and trying to follow the instructions here . I have gotten to:

Running cat /etc/os-release in terminal I get:

So what should I be putting in the deb command in the repositories list to track PlasmaC updates? None of the buildbot packages look correct...is it Debian only? Don't underestimate my noobness here!

Ah, finally found this thread, what's the recommendation? Can I just follow tommylight's instructions after the key server line?

edit: installing mint 19.3 since I'm messing around anyway...I'll just try to follow the instructions like a patient person...

I think I've comprehensively broken the software, otherwise I don't know what to think. I was briefly on 2.9/master but after getting the attached communications problem (error finishing read, and also joint following problems) I rolled back to 2.8 for troubleshooting. It's still showing as PlasmaC v0.170 (which I'm not sure if it should on 2.8), but I also get the error on the non-PlasmaC config.

Pinging the 7i96 looks reasonable (~0.2-0.4ms). Should I just clean install Mint 19.2 using BeagleBrainz's ISO?

Edit: I timeshifted back to this morning (Mint 19.1 / LinuxCNC 2.8.0 / PlasmaC 0.139), and hopefully it stays stable. I shouldn't have had this little 'adventure' on a Friday!

I've just tried 4.9 with the previous 19.3 / 2.8.0 and I still get communications-related errors (joint following error).

The joint following error and the error finishing read. I am now thinking I was borderline before, and maybe the change of version has made it more apparent (to the point it errors even whilst doing nothing) - so consider it a possible red herring. I googled it and people were saying these both point back to communications with the 7i96.

To date I've cut a single circle, and come to think of it I may have had a joint error right at the end.

What are the ping times to the 7I96?

What host hardware do you have?

Joint following errors and "error finishing read" errors point to Ethernet latency issues

0.2-0.4ms pinging from the terminal. The PC is a Dell OptiPlex 980.

OK that's fairly terrible...

That PC uses a Intel MAC, have you disabled IRQ coalescing? That's a requirement

Also that PC supports Intel AMT, that also needs to be disabled in the BIOS