Yeah, your tubing looks really convoluted. I have a very similar system, using the same distro plate, a CPU, GPU and 2 rads. I'm probably using half the piping that you are.
I can't quite bend my brain around your tube runs properly, but I think you have a problem at the top of your distro plate. Actually, if I'm reading it right, you're trying to implement the following path: Pump > CPU > top rad > GPU > bottom rad > pump
This is not how the distro plate installation is documented, and there are multiple demos on YouTube about how loop order is essentially irrelevant. The documented install path is: Pump > GPU > CPU > top rad > bottom rad > pump
I'll describe my loop, going clockwise, starting at the EK logo (bottom left):
- spare port, I have a temp sensor fitted
- out from distro plate, into GPU
- out from the GPU, into the distro plate
- spare port, unused
- out from the distro plate, into the CPU
- out from the CPU, into the distro plate
- out from the distro plate, into the top rad
- out from the top rad into the distro plate
- out from the distro plate into the bottom rad
- out from the bottom rad into the distro plate
Almost-illustrated below (this photo was taken when the temp sensor was inline in one of the tubes, not using a distro port). EDIT: the air bubbles in the CPU and GPU blocks are cleared now, so don't worry about them!
I suspect that if you follow your pipework you will see that you're causing a convoluted path for your fluid, which is causing the pump to struggle. EDIT: what my pic does do is clearly illustrates the chambers within the distro plate, which should help you to understand the natural routing of your fluid
Hope this helps