Some added evidence that supports the hypothesis that thermal throttling may be responsible for the flickering ending once it starts.
Shortly after switching on and going up to turbo/high, my IR thermometer reads a temp of ~70°c from the emitter. The temp drops somewhat once the flickering starts, which isn’t surprising. However, when the flickering abates a minute or so later, the emitter temp stablizes at ~65°c. This is not a result of the driver dropping down a full level, it’s still in turbo/high. If I do a quick press, the light wraps around to low mode.
I may try doubling up the tail spring later tonight.