Passat 3B: Excessive Smoke Under Load - Troubleshooting

Hello fellow diesel mechanics and experts, my Passat TDI is giving me a lot of trouble:
'At 2000 RPM, there's a noticeable drop in power with excessive smoke. At 3000 RPM, the problem disappears, and it doesn't smoke as much then. Oil consumption is not significant/minimal. No oil has been added since the last oil change 15,000 km ago. I also do NOT use biodiesel. Otherwise, it makes no unusual noises. It starts perfectly fine, with no issues. The mass airflow sensor (MAF) was replaced about 30,000 km ago and should ideally last at least that long again, because I need the car for my self-employment! Looking forward to your help. Thanks and regards, Lorenz.'