XC70: Engine Turbo issues

[Spriteracer]

Morning all. I need a bit of help please and apologies if this initial post is a bit lengthy.
I have a 2009 model XC70 with D5 and Polestar upgrade. Mileage approx 221k

The turbo was replaced by Volvo in 2019 (about 80k miles ago) Swirl flaps and actuator have been replaced and calibrated. A Volvo independent had a look at the boost pipes and couldn't see any leaks, albeit without a smoke test. EGR is clean and moves, Air mass device swapped for a known good one.

For the past few weeks it goes into limp mode intermittently with codes P029900 turbocharger under boost and P23484 turbocharger over boost at the same time. If the codes are cleared they can reoccur after 50 yards or 50 miles and can come and go driving along. The dash message is "engine service required - urgent"

Volvo have advised after two visits to diagnose, that another new turbo is required at a cost of £3k+ but are not 100% sure of the exact issue. They say all the relevant sensors are within parameter. Patently this is not realistic as it is probably more than the car is worth. Even an independent is likely to charge £1500- £2,000. The car is too good to scrap, but not saleable like this.

Does anyone have any further suggestions to offer, and can someone point me at a "how-to" to remove the turbo and it's controller myself to inspect. I'm reasonably mechanically minded and have built and run my own race car for many years.

Many thanks for any suggestions.