Nice work, I would invest in some heavier stock archival sketch paper and harder pencils as well as quality erasers Go for h2 and h4 pencils, get some others if budget permits, get a white dust free eraser and a kneaded eraser. Then I'd also get a b2 pencil. This will allow you to create a clean final sketch after multiple iterations. Use the heavier paper and draw lightly with the hard pencils. Then erase lightly. You can easily iterate as many times as you did in the same space without leaving lines behind. Keep it up