I have an '82 and '84 123, both of which pull strongly from low rpm, except when very cold. In 1985, MB made some changes to the transmission and rear end which resulted in lower engine speed at highway speeds. My 1985 126 (all '85 diesel cars had the same revisions) drives exactly as yours, gutless to 2500 rpm and then pulls like a freight train from there on out.
On the plus side, 3000 rpm translates to about 67 mph, versus 58-60 on the earlier 123's. Quieter at speed and better mileage are also good.
On the other hand, maybe it's not normal and we both need adjustments!
I have bypassed the boost cut-out aneroid with no change to the performance, and disconnected the boost feed to the ALDA with major change in a downward direction!!
Which leads me again to the conclusion that poor low speed performance is a trade off for the other items I mentioned.