Looks like the cheapest next step is to replace the radiator cap. If my coolant system isn't holding pressure and is boiling at too low a temperature, that could well explain the creeping overheat to ~220f and then sudden spiking to >250f as coolant boils.
Car is overheating and I can't tell if the alarming levels of coolant loss are a cause or a symptom due to the reservoir boiling over.