When I apply a scale to trees etc, I do so using the default 3D character as a reference. I set the scales on import, so they arrive as 1:1 scale in the packs. Of course trees etc all grow in size, so the meshes are flexible to make those adjustments.
So you set the project to use extend luminance range, of course everyone can use their own individual settings to reflect what they need. I use the Auto luminance setting to get an average balance for when I take screenshots. I tend not to create scenes, just a grid of the models using vanilla settings inside a ‘light box’.
Those screen shots look very good, sharp and with plenty of colour vibrancy 