I kinda did flatten my entire island but I'm going section by section. I only kept ramps if I needed them until I could destroy them, and the elevated lands I did keep are still going to be elevated lands. I closed up all my water sources too so I had NO RIVERS. I removed the one bridge I had (that didn't even go across a river, it went to a small island in the middle of a big pond).
As for tips, def go section by section it will be alot less overwhelming. Put EVERYTHING on the beach. Your house, villager houses, shops, as much as you can cause I don't think everything can actually fit on the beach all at once (I have 2 south facing rivers).
I literally drew a rough version of my town map on grid paper, drawing out the beach rocks, beaches, river outlets, permanent stuff like that. I have an album on my phone of screenshots of island inspo I wanted to use for my island. I drew rough area of where I wanted each thing to be. Because I didn't do all the math and go literal square by square, I had to be lenient with where I put stuff. I made one area bigger than I thought it would be, which causes me to shrink other areas to compensate.
ALWAYS have a ladder on you, esp of you want to demolish everything first. You will also need so, very, many shovels if you're redoing the landscaping as well. So many.
Thanks for the tips, very helpful! I already have screenshots of maps I've liked all looking very similar, so I'll probably base the terraforming off of those, but definitely won't be doing the square by square measurements, I'll just let my creativity lead the way. I'll defo make sure to have plenty of shovels on hand lol!