The Throttle body is easy to clean, the EGR system is another story.
The usual routine involves pulling the intake manifold to clear the blocked
port. There's the EGR solenoid and the line from there to the intake that was
clogged solid in my 850 at about 200K. The problem when that stuff clogs up
is that the crankcase pressure can rise to the point where the rear main seal
can and does fail, which is an ugly repair job.

I wonder if it's possible to clear the clogged intake manifold port via the throttle body instead of having to pull the manifold. Maybe use the little harbor
freight LCD boroscope to see what you're doing to clear the plugged mess from the inside-out. That might save hours of work? The standard routine is a guaranteed pain in the you-know-what.

Whatever you do, the 850 engines don't like dyno oil, with the oil clogging up in the EGR system and coagulating into a nasty hard crusty mess. Synthetic oil is better.

Another possibility is to bypass and/or remove the EGR system entirely. But I'm
not sure exactly how that might work out. Block the EGR port from the exhaust manifold and find a plan B to keep crankcase pressure in line. It looks to be worth researching.

Good luck, Bill