Only thing left to do without spending money on better glass (or strobes/lighting) is to just brighten the pictures up in post. That will increase noise, particularly in the darker areas.
To have the aperture set to the widest possible, select M and zoom your lens all the way out (55mm). At the widest angle, set your aperture to f/4, ISO all the way up and your shutter speed to whatever you want to "get by" with. When you zoom in, it will take the widest aperture it can as it is trying to set f/4 aperture. When you zoom all the way back out, it should revert to f/4.
You could take note of which focal lengths you used most frequently and try and buy one or two (consumer) prime lenses. Still going to set you back $$$, but not nearly as much as a 70-200L f/2.8.