Lumen reflections are more limited in some ways than standalone RT reflections, and quality is currently one of them.
To begin, the reason the artifacts dissapear when you look crossways to the mirror is because of lumen’s screen traces: basically, lumen uses a high-quality version of screen-space reflections as the first tracing method, and the detail problems go away because it’s reflecting exactly what’s on screen, and not the lower-fidelity lumen scene.
If the reflected area isn’t visible in screen-space, then lumen falls back to ray-tracing the ‘lumen scene’, a lower-res version of the world that’s cheaper to trace against. Lumen GI is high-quality for the first bounce and then uses faster methods for subsequent bounces, which is why the reflection of the scene lighting is so low-res. It doesn’t explain what the dots are, they don’t look like noise as much as a glitch in the surface cache’s scene parameterization.
Most of your settings are red-lined already, but I’ve noticed that lumen scene quality tends to max out at 8, so you could get a bit more quality if you wanted. Enabling hit lighting allows for highest quality as well, and lumen reflection quality scales up to 8 too, if you’d like to increase it. I’m assuming you’re using hardware RT with all of these settings.
Standalone RT reflections aren’t aware of lumen GI, so you’ll get a very strong lighting discontinuity. Planar reflections are very expensive and they have strange support limitations, and I’m quite frankly not even certain they’ll work with lumen. My recommendation, if your content allows it, is to use baked lighting and standalone RT reflections-it’ll pick up on GI, you can set the spp count to whatever you need for highest quality, and it supports multiple bounces.