Few things---------

If it heats up when you slow down you either don't have enough air flow or water flow-----in your case its obvious you probably don't have enough airflow

In your pic---------what kind of radiator cap---lbs pressure? and does it let the radiator refill on cool down????? probably lots of air bubbles circulating thru engine when warm-all new vehicles have closed cooling systems where expansion tank is mounted high in system---------

Head gaskets on correct way? if backwards and/or reversed they ll block/restrict passages and effect coolant flow

If it was in my shop--------I would remove all your serpentine system, and electric fans kit and install a normal rotaion, high flow water pump, etc and see how it ran-------- then reinstall your electric fans to prove them------

Make sure your cap is 16-18 lbs, and is for a closed system with an expansion tank mounted high in the system-------