Right well personally to me that sounds like the turbo isn't running effeciently, my guess would be the waste gate housing is cracked or the wastegate itself is damaged. Thats what controls the boost on the turbo and if your saying you are undoing the bleed screw fully and its still doing it thats about all it can be.

With the screw fully out the turbo should boost and boost and boost until the weakest part on it lets go.

You've got 2 ways of determining if this is the problem

Safe and annoying as fuck way:

Remove the turbo, and take off the exhaust elbow and inspect the waste gate, or:

the potentially dangerous but easier way:

Pull the pipe off the actuator and literally see if it will boost higher than it has previous, if/when it does stop boosting STRAIGHT AWAY. It's not guaranteed to work this way either if the wastegate is really fucked it wont work.

When mine was cracked i actually REMOVED the screw from the bleed valve before discovering the fault, and it would peak at 10psi but hold 6psi.