Hello GermanoGuidi,
Unfortunately, the effects quality is only for the entire project. However, there are a few things that you could try as alternatives. Each actor has options for changing the Detail Mode and the Material Quality Level, which you can find more information on here: Scalability Reference | Unreal Engine Documentation
Also, have a look at the FeatureLevelSwitch node, which you can find information about here: Utility Expressions | Unreal Engine Documentation
Have a great day,
Sean Flint