There is enough snot/scum/slime on that thermostat to tell me things are filthy in your cooling system.
If you plan to flush it out, drain the coolant & flush it with clear water. The drain it again & fill it with water and 16 ounces of Simple Green or Zep Orange, and if that is not available down there use a good commercial floor stripper like you would remove wax with.
Close the system up without the thermostat, and drive it for about 10 miles. Make sure you get the engine speed above 3000 a few times.
Then go drain the filth out.
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