Overheating Up-Hill

My girlfriend's Prius has been intermittently triggering the coolant warning light. It seems to only occur while going up-hill, especially under heavy throttle, though it has triggered under moderate throttle before.

I've watched the OBD2 temperature while this was occurring a couple times now thanks to the aftermarket radio doubleing as a scan gauge. The temperature is not a slow-steady rise, it quickly shoots up to the 280*C range over the span of 3-6 seconds, then shortly after letting off the throttle and coasting it quickly drops back down to the 200-210*C range that I normally see under sustained acceleration.

Most notably, this did not occur until after the car ran the coolant reservoir empty. We pulled off immediately and did not drive it until I was able to refill the coolant reservoir, so I think it's unlikely the headgasket blew from this event, but it is a possibility. Since this occurred, the coolant level has been slowly dropping over time, though it's possible some of the drop was from air-bubbles being slowly forced out.

I've already tried bleeding the cooling system once, including unscrewing the temperature sensor from the plastic block that connects to the EHR system, and lifting the reservoir up above the engine until coolant poured out the temperature port before screwing the sensor back in.

The temperature spiking and then dropping down in such quick succession rules out it being an accurate report of the cooling system's temperature, it's not possible for it to change temperature so rapidly. When looking online someone else who experienced something the theory was that an air bubble was moving around and sometimes surrounding the temperature sensor causing the readout to spike, but is it really feasible that an air-bubble would be 70-80* hotter than the coolant surrounding it?

Since the temperature reported over OBD2 is rising in combination with the light on the dash, that should mean the sensor reporting this data is the one on the back of the cylinder head, as the one in the plastic block under the wiper tray is wired directly to the gauge cluster, not to the ECU. (I suppose it could be grabbing that sensor's data over CANBUS and reporting it over OBD2, but that would be quite the odd setup). All of that's to say, I don't think it likely that the issue is stemming from the water pump as is often quoted in other similar posts.

The last thing that may be of consequence is that the car is at ~167k miles, and I'm not sure the coolant has ever been replaced. I'm planning on doing so shortly, but I'd like to sort out the overheating issue first so I don't waste $50 in coolant if the fix ends up involving draining the coolant. Would there be any likely culprit to check first for any corrosion induced issues that would have spawned from not changing the coolant on time?

I haven't spotted any coolant leaks, but the coolant that was lost initially had to have gone somewhere. Aside from the intermittent coolant light issue the car has been running perfectly, so I'm still doubting it's the headgasket that's leaking.

I've seen a few people here are adamant about periodically cleaning the EGR pipe, and I've seen it come up on a few intermittent-overheating posts. I'm planning on doing so soon, just need to order the gaskets first. Though I don't see how the EGR pipe being clogged would be causing this issue.

All of that's to ask, what's everyone's opinion on what I should chase first? I'm thinking:
1. Look more thoroughly for coolant leak.
2. Attempt bleeding a second time
3. Replacing ECT sensor.

Is there anything I'm missing? I'm not familiar with Prius's so if there's anything Prius-specific that I'm missing that would be good to know.