Coolant Temperature Issue

[RTD]

Hello, looking for help troubleshooting a coolant temperature sensing issue.  Background on the truck: 2002 Silverado with the 4.8L.  I’ve only had the truck a few months and after picking it up did a mini overhaul of it (new plugs/coils/wires, new brakes, front and rear suspension, fluids all around).  After getting all that done and getting it through a drive cycle, I thought the dash gauge was reading low and sure enough I got a P0128 code for coolant temp not reaching normal operating temperature. The dash gauge read about 170ish at the time.  I was dumb and didn’t get a scan tool reading.  I just suspected a stuck thermostat due to the age of the truck.
This weekend replaced the water pump with new thermostat as well as the fan clutch (no electric fans).  Drove it to work this morning and dash gauge temperature was about 10F higher than before (sits right under 185F now). Since I was watching it like a hawk I even noticed it burp up above 185 and then come back down, which I assumed was the thermostat opening up.  The thermostat I put in I believe is a 187F thermostat from Gates, so given some typical errors in the dash gauge I figured it was working ok.  For the hell of it I brought my scantool with me and checked the computer’s temperature when I parked, which said 153F. This doesn’t make any sense, and I’m concerned once the drive cycle is complete I’m just going to trigger the code/check engine light again (I’m in one of those unfortunate states that requires inspections with no check engine lights).
Any thoughts?  I find it hard to believe it’s a stuck new thermostat and thinking something else has to be going on.  I thought possibly a bad temp sending unit, but the symptoms don’t make sense to me for that either.  The truck does originally have the tow package which calls for the severe duty fan clutch.  Wondering if that thing is keeping things cooler than it should…