My beloved 2012 Plug-in is now my son’s, and he brought it back from college with nearly 200,000 miles. I cleaned the EGR cooler in 2018 and it wasn’t too bad. I’ve since installed an oil catch can. When we worked on it this week, I removed the EGR pipe, and it was very clean. Some stickiness deposited on the pipe but no build up, no chunks of anything at all. I also replaced the spark plugs and they were clean… just tan deposits, no oiliness at all. From this, would you say it’s safe to not dig into the EGR, which takes forever?
Do you have the car's EGR flow test numbers? Higher is better, around 21 or 22 kPa seems to be what people see right after getting shiny clean. (You might be thinking "kPa is a pressure unit, not a flow unit" and you're right; the car measures the EGR flow by opening the valve some and observing how much the manifold absolute pressure changes, so that's why the test comes out in kPa.) That's kind of the easiest way to know, because getting to the cooler itself to check is, as you've noticed, a lot of work.
