I doubt that is the intended setpoint, seems really low. As far as I know the 6.4 tstat is 216F or 217F. And they run em that hot for emissions, keep the cats hot for efficiency, at least that's one reason.
I run a 180F tstat, operates pretty solid at 190F did all summer. 190F on my factory cluster gauge is app a needle width below the half way point. Where you describe your indicator it could be up to 15F lower than mine.
Plus my factory cooling cycled from 205F to above 215F constantly. Rock solid seems strange, gauge could be dampened, I saw mine cycle on a digital monitor (T2) not the cluster gauge. It would be worth getting a temp gun and take a couple measurements and verify it's running at the intended temp.
Mine is a 4th Gen, maybe the gauge changed but I'd still verify, nobody likes warped heads or broken mani bolts.
Edit: since I started till I posted two others verified and towing that kinda weight, they must have slid that gauge a bit, probably so when we hit 280F the needle is only halfway and we won't give it a second look . Lol