"What would be a reasonably lightweight option for lower light situations."
Asked and answered.
Binoculars have a lens rating like 8x25. The 8 tells you the magnification factor. The 25 tells you how large the front lens is, and the larger it is, the better it can gather light. So, 8x50 gathers a lot more light than 8x25.
Camera lenses are similar. Typically the focal length is expressed in millimeters, like 400mm, and then the maximum aperture number, like f/2.8. The smaller that aperture number is, the better it gathers light when fully open. Photographers refer to that as a fast lens, although it really doesn't mean velocity of anything. If you gather more light via the wide aperture, you can speed up the shutter. Often that is good for wildlife.