I get that you're going for a lofi sound, but here are some fixes that'll make it hit harder without taking away from that. If you double-track the guitars and pan them to the sides, that'll give the vox more room. Add some chorus and delay/reverb on the vox to make them bigger, and check your instrument balance so that nothing's too loud or getting swallowed up in the mix (like the drums), and you'll get something heavier but still dirty.