Hi Ben...welcome to the Forum!
I'm not entirely sure but as you have replaced some of the parts that would affect the revs (or lack of them) I would see if it needs reprogramming with new settings for the parts (especially the pedal). On Nissan,you have to dial in the throttle pedal within a voltage range then it works fine otherwise, no revs.
Worth checking as a start, but don't also forget checking the Air Mass Meter readings and even blocked exhaust, EGR, etc...remember air has to enter and exit the engine for the engine to run efficiently.
Let us know how you get on with it. Cheers, Trevor