Engine temperature drops rapidly/slow warm up

Hello,

I recently acquired this vehicle and noticed the engine temperature gauge would drop on highway speeds and not reach operating temperature.
I have experience this issue prior so I took it into a shop for them to replace the thermostat. I was correct as the mechanic claimed it was opening early. They claimed they would use a quality thermostat as a replacement.

After the repair, the vehicle will now reach and maintain operating temperature and blow hot air. Great!

However, as it is getting colder I am noticing that the vehicle takes quite a while to reach operating temperature compared to a more modern vehicles.
My initial assumption is that the V6 simply takes longer due to the material used in the engine and its size.

Another issue I notice is that when the vehicle is at operating temperature I will go into a store for maybe 30 minutes and come back to the vehicle with the temperature gauge on C - cold, while my girlfriend's 2008 Saturn Aura V6 operating temperature stays in between operating temperature and cold (still warm).
Minnesota weather is currently around 30-40* F.

What causes my car to warm up slowly, even after a thermostat replacement?
Why does the car's operating temperature drop like a stone after such a short time being shut off?
Would a coolant flush help this situation?