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Fire it up with the radiator cap off and a 0-110deg thermometer in the rad. Just let it idle for 15minutes, or slip a sheet of cardboard down in front of the radiator to speed up the warmup. It shouldn't heat up at all, then suddenly get warm all at once. That's when the t'stat opens. If it is gradual all the way then the t'stat is stuck open. Some radiator shops will clean the block out by removing the radiator hoses and sticking a giant water-blaster hose in. You can do the same by removing the water pump while you have the rad out for a flush sometime and sticking a hose down the side of the bores. As banjo said, the dirt and rust builds up at the back of the block around #4 cyl.

The "block drain" LittleRedSpirit is referring to above, is a long hexagon tube type fitting, on K series motors, screwed directly into the block, just below the water pump, at the very front of the engine. It has a bolt head at the end, which acts like a tap. My experience is that on many K series engines, you undo the bolt head, and little or no coolant comes out. If that is the case, it is usually easier to unscrew the whole long tubular fitting out of the block. This is almost the lowest point inside the block water jackets, so lots of scale & crud build up, down behind there. By unscrewing the whole assembly, you provide a bigger hole & opening, for the scale & rubbish to get flushed out. Cheers Banjo

and if you replace the thermostat, test the new one before it goes in, they are often bad from new.

You need to remove the thermostat and test its opening temperature in a saucepan as it heats up. It may open early, late or not at all, or stay open if faulty. Judging by the fact the car doesnt show super hot on the temp gauge, or seemingly up to temp, theres a few things that can xause this. Make sure you have accurate temp readings, so try and install a new temp sender for the dash gauge, or still better, install an aftermarket electronic gauge like a speco so you can talk in real numbers and not in hot cold or the red bit terms. If you think its working ok, maybe just remove and descale the probe on the gauge and make sure its clean enough to absorb heat from the water directly. Now that you know your temperatures for certain, i would remove and test the thermostat as described above, usually if a car fails to get fully warm its because the thermostat inst correctly regulating the flow of 82 degree water. Basically, the thermostat has a wax substance inside it that expands when hot to force the thermostat open, they are designed to open at a certain temp, and close below that. Basically your thermostat is probably staying open, so Id suggest a simple replacement of that, and any physical removal you can do of crud will help the cooling efficiency. Most Toyota motors have a block drain where you remove one bolt and all the coolant runs out the bottom of the water jacket. If you remove this bolt on a full cooling system and little to no water falls out, then its will be pretty heavily scaled up inside and it will benefit from being picked out with a tool so it drains again and even try running muckowt for a while. Motors perform at less than their best when run too cool.