I actually just started this same project, and have my RTX A2000 running with an i7 8700 (non T) in a M720Q with no crashing and CPU at 75W peaks.
I don't think you can wire into the shunt with a parallel supply otherwise you risk damaging your motherboard or card. I did a lot of reading into this and though it might work if the voltage on your auxiliary PSU drops it will instead take the full draw from the slot and possibly cause damage.
I currently have a flexible PCIE riser with the 12v and their respective ground lines(disconnected from the PCIE slot end) soldered to a 200w PICO PSU and it all works as normal though now more cumbersome, I'm hoping to get a different GaN PICO PSU to tap right into the 19v port to power it and sync it to the M720q PSU with a 5v relay in parallel on the Sata port. There are other 12v/5v points to tap into but this seems to be best for modularity. To actually put it back together as a sealed unit I'm going to design a custom heatsink as I was planning to replace the aluminium one anyways. Hopefully i can shift the CPU fan back enough to put the PICO PSU at the front of the case and solder the 12v line to the back of the riser after severing the 12v and ground traces on the PCB. Unfortunately there isn't enough power budget on the board to pull a usable 12v line to help power the GPU
