My 2015 Ford Ranger is overboosting at idle and only revving to 2000rpm

Since you’ve replaced the turbocharger and the boost controller, we can rule those out. But could the problem be something much simpler?

It’s possible that the plastic trunking that takes the compressed air from the turbocharger to the engine’s intake has split. When that happens, boost pressure is lost, the boost sensor detects the low boost and winds up the turbocharger to compensate and you have an on-paper overboost situation.

Then, once you’re beyond idle, the leak becomes even worse and the computer simply runs out of turbocharger capacity, at which point boost drops away. That’s preventing the engine revving properly and since there’s no sensor to tell the computer that the trunking is split and leaking, there might not be a fault code issued. Lots of black smoke from the exhaust is often (but not always) another clue that this is what’s happening.