Although we can understand that you would like to edit roughness maps with more control in the Enscape Material Editor, we rather want to focus on keeping the UI as easy as possible. Introducing multiple sliders per map would have the contrary effect. Additionally, visualization experts are still able to adjust their maps outside of Enscape as a workaround.
ok then keep only one slider, but it should be set in the middle. Moving it to the right should increase brightness and moving it to the left should decrease brightness. I use enscape because it is fast, any workarounds in other image image editors are slowing down the material setting process. It is basically back and forth (changing the brightness, levels, gamma, saving image, loading image) checking how it looks in enscape. Therefore i think it should be built in enscape to have immediate visual control over the material settings process.