Hi all, thanks for great guidance.
I have now replaced all regular springs with a kit from Wolff. I found a regional dealer in parabellum parts who sold me a FP with the relief cuts as well as a proper old style spring retainer.
Replacing the mainspring was tricky at first but worked fine once I read Thors guide. (Thanks Thor
)
I also tightened up the sear spring a bit to improve Sear+FP hook-up.
After this the trigger pull is a lot heavier at maybe 4-5 pounds, but manageable. Not much creep but a slight increase in pull right before the break. I will do some polishing to mate the surface between the Sear and new FP better.
Now the gun happily feeds all 9mm ammo I give it including my slightly downloaded TC rounds.
Using tape I noted that production loads like Magtech 9B and S&B V310452 slams the frame even with the Wolff standard strength spring. I developed a softer load with 4.5gr 340 behind a 124gr TC plated bullet that does not mark the tape yet cycles the gun and makes relatively accurate holes in the paper target 25 meters away.