Thermostats vary coolant passage over a range, and are not a snap-on/snap-off situation. Per the service manual:
Starts opening: 169-176°F (76-80°C)
Fully open: 194°F (90°C)
That more or less fits with your observations.
You DO NOT want to install a "cooler" thermostat. The K24 is not a hunk of sloppy Detroit iron, it is a tightly-spec'ed alloy design, which mandates normal operation within a specific design range. Even lowering the range by 10°F will likely have adverse effects on fuel economy if not engine life.