Setting your ‘Skylight’ will give you the highest quality and highest mutability.
I would also look into adding sphere reflection captures instead of the box reflection captures. If you take a look at the documentation, which it seems you are following to an extent, we use sphere captures and light overlapping areas to get the best detail from the reflection environment.
Reflections
Try the sphere reflection capture actors instead, and changing your ‘Skylight’ to movable to see if that improves your reflections.
Cheers,