"super old, heat-cycled out r-compounds"

They aren't the same tires you used "last track day" they are the same tires you used half a year ago. There is no point in comparing how your car behaved 6 months ago to your car now with an entirely different setup and worn out r-compounds on a day that likely saw different weather and track conditions. I am sure you can tune out the understeer with the current worn out tires, but you will likely have to start fresh when you get a new set of tires.

Drive more often!