I ordered some cool wire block terminals because the terminals on the PSU looked too small to be able to fit 2 ferrules into them. Other than having to double up the 20AWG silicone wire for the live bed wire, they were no problems with them. They're quite easy to use; just strip the wire, insert and close the lever.

Mechanical relay issues

I failed to integrate a mechanical relay into the AC bed circuit. The idea was that, in the event of a fault, AC power to the SSR can be cut. This is because I've heard that SSRs can fail in a way that keeps power permanently on.

The wires I tried to use were fairly thin and my smallest size of ferulles (white) were not small enough so I had to fold over the bare wire and then crimp multiple times. Additionally, these blue terminal blocks are awful as the white ferulles only barely fit and one of the screws had quite a bit of friction. I'd recommend finding a relay with green blocks:

In any case, the 5V relay, in either the high or low trigger mode, didn't seem to close the connection. I'm not sure if I'm missing anything in the Marlin PSU-on feature settings, but I think the reason is hardware related as the signal pin uses 3V3.

Thermistor fluctuations

I cut off the dupont and crimped XH onto the wire that was already there, and all seemed fine other than I was fluctuating between 21 and 23C degrees. I thought it was a poor crimp job so I tried again (and one of the crimps literally came undone just trying to get the wires out of the housing). Second attempt had no thermistor reading whatsoever so the 3rd time I just soldered on a wire with a pre-existing XH termination. Still got the fluctuations, just at a slightly higher frequency. It'll be something like 24->23->22->24->23... at a frequency of 0.2Hz.

Conclusions

I should finally be good to finally answer the main question: Does the #Coaxial8or [gd0144] work as intended? All CR600S related systems seem operational now.