Unfortunately? I believe you’re out of luck on lumen reflection quality. Long story short, lumen essentially can’t handle glossy reflections as a usable quality level right now.
Longer story: Near-mirror reflections are very coherent, which makes them cheap (relatively) and easy to denoise. Rough reflections (.4+) are incoherent enough that lumen can interpolate their lighting from the diffuse GI probes, so they’re cheap and stable. But glossy (.2-.4) reflections can’t resolve well with the current denoiser and are really unstable, which is all the boiling and flickering. Weirdly enough, raising the ‘lumen reflection’ quality setting seems to make the noise worse, and Daniel Wright mentioned glossy reflections being a big priority for them to improve.
That being said, you do have a few options on how to solve this problem (if dynamic reflections are a requirement):
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Adjust the r.Lumen.Reflections.MaxRoughnessToTrace value to something lower than .4(default). This will force the GI probe interpolation to kick in earlier, and while you’ll lose detail on your glossy reflections, they’ll have less noise and the smooth ones are unaffected.
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You can also do the opposite, use r.Lumen.Reflections.SmoothBias to force all materials to be smoother, and therefore render more coherently.
Obviously, both of these will change your art direction, one way or another. But assuming the scene requirements are for a real-time game that’s using dynamic lighting and GI, I think these are the two best knobs you have at the moment. If anyone has anything else that works, I’d personally love to hear it.
Best of luck to you.