Okay, I am officially an idiot.
Went to run those tests today, only to realize that when I re-installed the EGR valve after cleaning it and replacing the gasket last week I installed it with the connector pointing towards the rear – exactly backwards. Switched it around so it’s actually installed the correct way and it immediately resolved all the problems – no performance issues at all, no codes either pending or current, even after spending the day running the car through a couple of complete drive cycles, etc.
Looks like I get to chalk this one up to the mechanic (me!) not paying enough attention and doing something colossally stupid. I’m just glad it didn’t damage anything major, and I caught it before I wound up having to spend a ton on diagnostics. I did go ahead and walk through those tests, after I put the EGR valve back on correctly, and everything tested fine, no problems.
Thanks Cig!