Opinions on smoking engine?

I’ve had good service from my D2-75 over the years, but I regularly face turbo clogging issues because as a full time international cruiser I prioritize fuel economy,  so I cruise at lower RPMs (~1800) rather than the higher RPMs advised to keep the turbo spinning freely and the exhaust system clean. Occasionally, I’ll run the engine up to 2600+ RPM to burn off soot, but probably not often enough.

Previously, I could achieve about 2650 RPM without smoke, even when suspecting a stuck turbo, and around 2750 RPM after an elbow/turbo cleaning. However, it’s difficult to know the exact state of the turbo without physically inspecting and cleaning it. Perhaps it was spinning after all?

This season, the engine started smoking at RPMs above 2000, and it wouldn’t exceed 2100 RPM regardless of throttle input, though it ran smoothly at lower speeds. Suspecting a clogged turbo and exhaust elbow, I inspected and cleaned both. The exhaust elbow cleaned up easily, but the turbine housing was rusted and the turbine wouldn’t spin freely despite thorough cleaning and rust removal. The wastegate was also completely seized, and I could only force it open mechanically. After reassembly, the engine now reaches over 2500 RPM before smoking, confirming the reduction in back pressure helped.

I’m puzzled about why I can’t surpass 2600 RPM without smoke. Two possibilities come to mind:

1. My turbo was actually spinning freely in previous years, even when I suspected it was stuck.

2. Another restriction elsewhere in the exhaust system is causing excess back pressure, though visually the manifold inlet appeared clean.

Any insights or similar experiences would be appreciated.