Fairly common problem for diesels. I have the same engine in a q5 and I had the same problem. I opted for a flash tune, disabling the EGR system (the cause of sludge buildup) and removal of the swirl flaps which just act as a surface for buildup.
Your mechanic is right, not much you can do without physically cleaning the internal parts of the intake. There are some products you could try from Supercheap etc that will try and clean the intake but they are unlikely to improve the situation much. If you have the time, tools and patience you could tackle the job yourself.
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