The biggest problem is the pcv system and egr. Both recirculate exhaust gas or crankase blowby back into the intake system
This is what cruds up the valves, hence the commercials for gasoline that cleans intake valves
Since we do not have the ability to "wash" the valves. It cakes on layer over layer. Google direct injection intake valves and check out the photos. I've done mine twice in 50k
Symptoms are basically nothing more than reduced power due to airflow restriction. However in your case with so many miles I would be worried about flow imbalance as they are probably almost clogged lol
I used a walnut media blaster to clean mine. Half hour to remove the intake manifold, an hour cleaning and then a half hour to put it back together
How about the sparkplugs? Done a compression test recently? Just really curious