As far as I know you cannot get high quality mirror reflections with Lumen. You can get much better quality reflections if you enable hardware raytracing with Lumen, but you can’t get high quality mirror reflections.
Hopefully this will be improved in the future, but that is simply seems to be the reality right now. If you need high quality mirror reflections, your only real option is to use Lightmass (instead of Lumen) with raytraced reflections or planar reflections.