Hey Maximvm -
As you say this is a complicated issue, but you could start by using a Blueprint to blend between a material using your render camera and one in which you are using a reflection vector on a highly metallic, no roughness material. You can then interpolate between them based on the player’s distance. This way your mirrors would never lose reflectivity but you would be taking the less costly route for objects farther away and the more detail, accurate route close up (So, your are developing an LOD system for your reflections).
There are still some more issues, but this should get you started.
Thank You -
Eric Ketchum