Cross posting here, hope that's ok:
I went from exhaust/push to intake/pull and I got an extra 5c. I was skeptical but having now tried both, it's all clear now. You do wanna run your slim noctua as an intake on top of the 645lt radiator.
I also installed this heatsink onto my rear m.2 on the phantom x570
It is helping, but i still idle around 50-ish c. I'm not gonna worry about it unless someone says i should. I haven't stress tested yet but i was at about 60c while playing valorant.
I did read about using 3mm thermal pads to the back case but i didn't try that yet and unless i need to i'd rather not pull apart the build AGAIN : P
I also moved the gpu riser cable to the back of the gpu instead of back of MB to make room for the heatsink and I installed an intake fan under the MB by further bending the riser cable.
xo,