To be honest mate, and with all due respect, you are potentially going to kill the turbo if you keep haphazardly guessing at it like this.

The first thing to do is get the whole inlet pressure tested to 30 psi as this fault sounds like an air leak developing at about 15psi. The most common cause is the dump valve, especially if it's a Bailey one, we take those of all the time for leaking at about 12psi.

With an air leak, the turbo will spin faster to overcome it, but it is trying to pressurise the whole universe so it will die trying.

Bleed valves are fine fitted properly and allied to an actuator that has the balls to maintain the bulk of the pressure. IE: bleeding a 5psi spring up to 30psi is pointless. Your fighting turbine inlet pressure too so you need a spring ,an enough to offset that first. For manual bleed vales I recommend the turbo smart one we sell, I've fitted them to tens of cars and they work very well. Even a normal brass 3 port valve will work fine if it's set up right.

As an aside. To quickly see what the turbo itself is capable of, just pull the pipe off the actuator and block it then take the car for a gentle full throttle run in 3rd. It will now run whatever it can do until turbine inlet pressure overcomes the spring as it has no pressure assistance above the diaphragm. Sometimes the wastegate has problems or a big air leak is hindering you. This will quickly help to prove this is the case.