Why did you put 8.1 on before win 10? you didn't need to go through win 8.1. It makes no difference now but is a step you didn't need to do, in most circumstances.
If PC won't start with GPU attached, it could be your power supply or GPU
what GPU is it as I can't see one in base model? https://www.cnet.com/products/dell-xps-8700-mt-core-i7-4770-3-4-ghz-16-gb-2-tb/specs/
Does dell have onboard display connectors?
I doubt the dell PSU is top quality
Your original error is likely caused by old drivers, doesn't seem to lead to a power problem exactly