I made up the design for the 12VHPWR cable issue, not a perfect solution but I am confident it will prevent damage to anything.
Basically, I will connect a thermocouple to this very clever device from Aquacomputer that I use to measure coolant flow (and a few other things) in my water cooled rig. Then I will fasten the thermocouple to the plug of the cable at the GPU side with some black rubber bands.
Right now the sensor is only measuring air temperature in my room but it's supposed to be a contact thermocouple. I just hooked it up to test it and it works, the Aquacomputer high flow NEXT makes a loud noise and red LED illluminates on top of it when it alarms out. It even displays what the alarm is about.
My 5090 is still on the way, but I will report back with real test.