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The first thing I would do in your shoes is pressure test the system to make sure you do not have any leaks. Then I'd make sure your filler cap ("radiator cap" in the rest of the world) is fresh and seals properly under pressure and vacuum, and make sure the overflow reservoir cap is good, along with all its hoses.
If things check out so far, I'd do a chemical flush. Following that, I'd fill with water and drain from every drain port (radiator, engine, and pipes under the car) several times until the water runs clear. Then I would replace the thermostat and gasket with a new Toyota part, fill with fresh coolant (preferably red Toyota coolant since you will now have a perfectly clean system with no green coolant left inside), and bleed the air out following the factory procedure. I like to idle the engine with the heater set to the hottest setting while I am burping the system to make sure that the coolant is circulating through the entire system.
Thermostats are one of the parts where OEM seems to be far more consistent and reliable than aftermarket parts, so it worth getting the right one from Toyota. the inconsistent temperature readings you are describing are classic symptoms of an aftermarket thermostat. The same thing goes for the filler cap; aftermarket versions, like Stant, are just not as well made and often do not seal properly or last very long.
Here is your cost through us on those parts, should you choose to replace them:
90916-03090 - $14.87 - Thermostat
16325-63011 - $4.07 - Thermostat gasket
16401-63010 - $11.08 - Coolant filler cap
They're added to the same quote.
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