Again, the fact it’s producing the same velocity at a variety of different hammer spring settings means it is operating on the plateau. It is uselessly blowing air out the end of the barrel after the pellet has departed. BTW it is not necessary to swap out the hammer spring to know this to be true.
The only other possibility was that perhaps the threads were stripped and the adjustment wasn’t actually changing the spring tension, but the fact that you were able to take it to coil bind removes that from the equation.
To increase the velocity, you will need to either increase the regulator’s setpoint or increase the porting, or both. Short of swapping parts (e.g. longer barrel), there are no other ways to do it. Note if and when you increase the regulator’s setpoint, it may then be necessary to use the stronger spring…but only if you are unable to reach a point where adding spring tension produces no further increase in velocity.