You mention it runs 180° moving. When you come to a stop for a light for example, do you recall how long idling before it hit that 220°? What temps do you normally see at idle and for how long idling to reach generally speaking?
So it runs down the road at 180° with a 180° thermostat (is it a high flow stat?) Ever try a 160° thermostat (high flow) to see what it’ll run down the road at? What if it ran at 170°? (for instance) it may still run at 180° eventually, who knows. Obviously if your 180° stat is high flow and you replace it with a 160° high flow stat the temps will still climb the same amount at idle. Might take a little longer idling to eventually reach 220° might not? It is possible if it runs down the road 10° cooler it might only hit 210° at idle within the same timeframe as in the case of as you mentioned “at one point 220°” and may still climb.
Still not ideal, but certainly not as alarming or concerning. I rule out nothing, experimenting never hurts