keeping the wastegate closed is a good point becouse it starts opening once you have boost.
0,5bar at 3400rpm, i dont think you will ever get nice spooling with this turbo on your 1,9. I think the turbo is just to big.
its my understaning you have no big ristrictions in the intake and at least a 2,5" exhaust system, and tried more lda settings.
you can try to increase offboost feuling quantity (keep egt & smoke in check), playing some with IP timing, alltough its not gonna do much if you wanna keep it safe. installing a exhaust-turbo-wheel with more blades or other turbostuff where you need a turbo-rebuild for...

Greetz, Benjamin