Function of the VTG control on the VW T5 TDI (AXE)

Hello, diesel mechanic community!
I'm new here and would like to introduce myself. I drive a 2004 VW T5 TDI with an AXE five-cylinder engine. As an "enthusiast who likes to tinker," I try to maintain it in good condition as much as possible, provided I have clear instructions for the repairs.
The last major repairs included replacing the camshaft along with its small parts, and replacing the dual-mass flywheel, including all related components. Clutch and a few other minor repairs to the chassis.

"For some time now, I've been experiencing problems with the turbocharger pressure exceeding its limit." I tentatively replaced the N75 wastegate control valve, but that didn't work, so now I'm going to try replacing the mass airflow sensor. However, I still need to do a test drive, which I'll do today.

However, I lack the basic knowledge of Turbo that is needed for troubleshooting.
How exactly does the control system for the VTG loader's buckets work?
Sensors capture measurement values -> the N75 control valve regulates the vacuum pressure on the turbocharger's wastegate actuator -> an adjustable linkage rotates the lever of the variable geometry turbo (VGT) adjustment mechanism until it reaches a maximum position, which can be adjusted with a set screw.

Roughly explained.

I am at least unsure about the following:

-At which lever position is the turbocharger pressure reduced?
-On the driver's side, a boost pressure sensor is located inside a plastic tube. Is this only for emergency use, or is it also used for controlling the turbocharger boost pressure?
-Do the turbochargers have a wastegate valve? I hear a distinct hissing sound when the turbo kicks in, which might not have been there before the overboost issue. The intercooler system is now sealed after numerous repairs. I still need to check if there's any exhaust leaking from the turbo's control shaft, using a soapy water solution. I don't see any soot.

I would appreciate some basic information about VTG adjustment, specifically what the maximum and minimum positions are.

Thank you in advance!

[Translating...]

LMM

Boost pressure temperature sensor.

Control valve.

This error also doesn't provide any helpful information from the language model.
You probably have a VTG (variable geometry turbocharger) that is sticking, or your boost pressure sensor is showing incorrect, high readings.
Regarding the minimum and maximum positions: high maximum boost pressure (at idle), low maximum vacuum at the VTG valve results in low boost pressure. Therefore, the turbocharger pressure is reduced in the same way as when the engine is not running.

Good news!
The persistent issue of the engine constantly running in a high-revving state suddenly disappeared after the mass airflow sensor (MAF) was installed AND...
Probably the cause now:
I noticed that the VTG adjustment can be moved smoothly by hand against the force of the spring in the vacuum chamber, even though it requires some effort.
BUT there was a "sticking" or "jerky" effect when pulling towards the zero position.
I then manually moved the lever back and forth from the zero position to the maximum position several times until the clamp was released.
That fits with your explanation!
Idle state/no vacuum = low boost pressure.
The boost pressure doesn't quite reach "zero," which means it will eventually become too high.

Now I'm curious to see if it was just a bit of soot that was causing the problem, or if the VTG mechanism is worn out and sticking.
I am observing...