Did you remove the FPB and spring, reassemble pistol and try trigger? If not do that.....need to completely rule out the FPB. If you still have the 12 lb. break, put the FPB back in, as its apparently not part of the problem.
Refer to the picture below:
Oval A is the single action lever, it props up the single action sear, which is Oval B. When you've moved the trigger all the back to the wall, a tab on the trigger bar (that we can't see in this particular picture) just made contact with a tab on the single action lever. THIS is the point where, the pull leading up to this point was normal, and the little bit of trigger movement from this point results in a 12 lb. pull/break. There are only two things moving once you're at the wall and continue the pull .... 1. The trigger bar moves straight back and 2. The single action lever is pushed back about 1/16" to maybe an 1/8" (eyeball measurement). So, the trigger bar is binding, hit a ledge or something or the single action sear is just REALLY heavy (which it shouldn't be....it should be very easy and smooth as silk.
So, if removing the FPB had no effect on your 12 lb. break its time to look elsewhere. Refer to the picture below. Remove the slide, put your thumbnail on the single action sear at the point indicated by the green arrow. Apply constant downward pressure on the single action sear....while applying pressure, pull the trigger back until you're against the wall (go slow, you'll feel it). Once you're at the wall, go ahead and continue pulling the trigger. You'll notice the single action lever moving rearward as the trigger bar moves rearward....it won't move much and you'll feel the single action sear 'drop' due to the downward pressure you were applying with your thumbnail.
The question is, was the pull at the wall 4.5 lbs or 12 lbs.? Was it smooth? Did you see the trigger bar jump, as though maybe it just went over a burr in the side of the sear housing????