I would take a look at a couple of settings in the Post Process Volume.
Check for Raytrace reflections MaxBounces, maybe about 3 will help.
Samples Per Pixel if you are able to can be increased a bit to help with the flickering.
Also check out the cvar r.RayTracing.Reflections.ReflectionCaptures = 1, it will use reflection capture actors in your scene to help fill in some data for reflections on the last ray bounce.