Personally, I would not be able to stand flattening my entire and starting from scratch. It'd drive me nuts, I'd get overwhelmed, and I'd most likely end up starting a new island, lol. When I picked my island map I SPECIFICALLY chose one that had a natural layout I already liked. I've definitely terraformed some, but never more than one area at a time. That'd be too much for me. So, again personally, I wouldn't flatten everything at once. I'd do it piece by piece. Say, finish the museum area first, then the shops, etc. (in whatever order works best) That also has the bonus of, if I change part of my design during the process (which happens ALL the time) and have to re-do work, I have less work to re-do if it do it piece by piece.