I know it sounds daft but if the turbo was knackered would the car still drive fairly normal??
It can be driven like you stole it but it just doesn’t feel like it did when I bought it.
It was rapid and pulled hard through all the gears now it’s a bit like all exhaust noise and not as much go if that makes sense
That is unfortunately the symptom of a turbo on it's way out. Don't forget, turbo engines totally rely on the snail to make any kind of power. If that goes down, so does the power. The turbo obviously hasn't binded to the housing and is spinning enough to move the car, but clearly not what it once was.
It may not be completely dead as in smoking and making grinding noises, but it could just be the wastegate is not sealing properly, so the turbo is doing less work, which means more lag, and less power. Turbine wheels act like mufflers, so if the wastegate isn't sealing properly, it makes sense bypassed exhaust pressure is increasing exhaust drone.
I could be wrong, but the snail is where I'd start my investigating. Drop the oil and inspect it and the filter for shiny brass coloured flakes, which could be the turbo bearings breaking up. If the oil and filter are clean, it may just be the wastegate but it's on the edge of making the requested boost, slowly, hence no 'under boost' codes.
