A place to politely discuss the tools, technique and culture of photography.
This is not a good place to simply share cool photos/videos or promote your own work and projects, but rather a place to discuss photography as an art and post things that would be of interest to other photographers.
Brightness in photos is arbitrary, and has nothing to do with how bright the scene actually is. Instead, it is a matter of processing.
Sure, you need to figure out how to make the light appear to be coming from sunlight through a window, but a flash can simulate this. Adjusting the white balance to make the light appear yellowish is easy as well.
Use gels to create color differential. Sunlight should be warm, shade should be cooler. Not much, just a little will make it feel natural.
Big defuse light sources, light from windows is very soft. Light through a cheap sheet or bounce off a big wall off frame.
Bounce the flash off the wall next to the window or next to you. If there are no windows in the shot, it can appear as light coming from a sunny window.
Direct sun is the definition of hard light.
Soft boxes and diffusion are working against you.