Hey Luciferr, glad to see you back at it!
First thing I’d do is disable all GI/FG as they have a tendency to mask what's going on with the direct lighting. Just like modeling, you block out a rough version and refine in phases. Area lights are great and quadratic decay the closest to real world light falloff. Right now the entire gym is being fairly evenly lit because the light isn’t decaying, so it’s flattening out the scene. Adjust the lights to have the desired temperature/color, then you can tweak the intensity and balance that to create the mood and turn on GI/FG and tweak further. The decay also helps to focus on a subject, for example, the big bright lights can highlight the boxing ring.