First I will say that I am in no way a boat builder so I could be completely out of my league here, but going off of what @COlden and some of what you were saying as option #4, you can take an approach similar to this project, and using 1/4" ply or even 1/8" hardboard, you can get design the shape you want, slice it up like the project I mentioned, and just figure out your spacers for in between to create your “skeleton,” then use the expanding foam to fill the spaces. The hardboard can then be your guide as you sand down the foam to the proper shape, then fiberglass the outer shell of it. That should give you the lighter weight you were looking for, as well as some structural support. this way, all you are doing is cutting flat surfaces and don’t have to worry about the 3D contour milling. Just a thought (please, if I am wrong about this, someone tell me)!
Good luck either way!!