Crank up the gain on the camera.
I don't really see why high gain can't partially offset a slower scope.
But also, post an image and maybe a screen shot of your histogram before and after a stretch? And maybe hit Centaurus a instead of M83 while dialling in systems. M83 is relative bright as far as galaxies go, but is still bloody faint, even in an f5 scope it takes a lot of integration to see much. You'll see what's happening and what needs fixing quicker with cent A if it has to be a galaxy, but otherwise Carina neb will throw up results quickly and you'll troubleshoot more quickly.