If a sleeping bag was waterproof, you'd end up with condensation inside. It'd have to be goretex or something similar to be breathable and would more than likely not be 100 percent waterproof. It'd cost a small fortune also. My bivy sack has too much condensation when closed up, but my bivy sack isn't that good.
Your tent is light enough. You could buy a lighter sleeping bag. Using a tarp could save a little weight. I used to cowboy camp and skip the tarp unless it was likely to rain. I hammock camp nowadays because it is way more comfortable.