Congratulations and sure sounds like you've mastered the DTS. Sadly, I'm at a similar point with Park Avenues, the list is long on what I've been forced to fix either due to failure or just pm items. Currently on the second one in two years and have learned quite a bit about the common and uncommon failures and maintenance items. Love em on the open road and 26/29 mpg is common when running 70 to 80mph.
The above said, I am curious about the DTS's, I do recall issues with the earlier Northstar engines, I'm guessing the headbolt problem was finally solved? Also when looking for a car, what are the "this is a good one" clues, after the obvious easy to see details? With Park Avenues, you want to see a Carfax showing the transmission fluid and antifreeze being changed on a regular basis as in every 3 or 4 years. Rare but they're out there.
Looking Good and Good Luck!
RonR