This is a visual description of the changes I wanted to do to the loop. The horizontal line (blue marking) had to be replaced as it had a leak. It was also to be changed to run just above the PSU, in order to fit into the new bottom GPU.
The green marking shows that I wanted to lift the reservoir and pump, as that was the only way to make the horizontal line connect properly.
The T-junction with the outlet ball valve (yellow) was due to move to the other port of the pump top, as it was logically lower in the system.
The red markings showed a proposal for new tubes, the top one leading from the case inlet port to the top of the res, and the bottom squiggly one going from the ball valve to the outlet port. In actuality, the outlet port was placed on the front side of the case (behind the front panel), and not on the bottom. It saved me two soldered joints and an impossible cut in the case.
Testing the new card before switching to a water block.
Permanently installed outlet. The pass-through plug that's fitted to the chassi makes it incredibly easy to just screw in a fitting with a hose and turn the valve when it's time to drain. No spilling inside the case anymore!
Beauty shot. Second GPU hooked up to the loop, most of the pipes were done again to fit the changes to the system.