Cable or rod connecting vanes to the accelerator coupled with a wastegate to bypass the turbine and control max boost is workable. The downside to that approach is that with the pedal floored, the vanes will be in their closed position. At higher rpms the restriction of closed vanes will significantly impact maximum power. At higher rpms with boost controlled by vane position instead of wastegate, the vanes will be positioned in a very open position allowing excellent flow and considerably more power for that boost pressure.
ROR, with my system, the vanes are only closed at full pedal until boost rises. Steady state full pedal will result in the vanes being as open as possible while being able to achieve max boost.
If you installed a stronger spring in your wastegate actuator, you would increase your max boost.
For turning VNT actuators into boost cans, I have used micro nuts and bolts around the perimeter to make them resealable without messing with crimping and un-crimping. I have gotten stronger springs from McMaster and then cut them to size to dial in the boost pressure. You also have the option of adding an MBC to the boost can if you don't want to mess with the springs. With MBCs, tho, I've run into the issue that the smaller VNTs build boost so quickly that an MBC cannot react fast enough to regulate boost and so it can spike high and then drop back to the control pressure. Using LARGE hose and fittings can help with reaction time.
Yup, that's why I went to the stiffer springs. You could also add an external spring which would be easier to swap. It just has to act opposite the boost pressure.