My car returned from the shop this week. It turns out it was the thermostat that was faulty. It was stuck in a “closed” position, meaning that despite me topping up the coolant, not enough of it was circulating through the radiator to cool the engine.
With labour and parts, the total repair bill came to $375, which isn’t cheap but not terrible either. Unfortunately, since the engine was run in an overheated state, there is a potential for the head gasket to have been damaged. Such a repair for that would likely be in the thousands. I can only hope for the best going forward.
As for the car now, it runs fine and the temperatures are once again in the normal range.