My Model 66-6 is a 2002 "factory test shot" (May 28, 2002 to be precise from the fired case in the serial numbered sealed bag that came with it).
Mine is a 4" and with factory Uncle Mikes which are actually quite pleasant and buffer most of the felt recoil.
I run factory ammo...no handloads, usually Magtech 158 gr SJSP, or (more common when carrying) Remington HTP 180 grain SJHP both supposedly at about 1235 fps.
I have not had any loosening or bad effects show up at all. Like yours, mine has the last of the one piece heavy barrels, and the adjustable sights means that my Model 65 (basically same gun but fixed sights) is now mostly sitting in the safe unexercised cause me old 73 year eyes just pick up the red insert and the white outline so much quicker and get POA and POI to match up.
Of course I do have 1 feature that you don't have (Long sigh) and that is the infamous, dreaded, (yes still have the 2 keys) Infernal Lock.
So if anything starts to go on my 66 I'm guessing it will be some malfunction with the IL, which a "lock delete" kit available on this Forum will correct.