My thumb, index and middle were all very flat, thin, and hooked when I started growing them out to play guitar a few years ago. For me the ramp shape made the problem worse, because it practically eliminated one side of the nail and thus the strength and curvature of the nail. This meant constant fixes with China silk or fake nails. I would often have all natural nails for a week or two but then I would have to do one or more repairs. Now the thumbnail on my right hand has taken on a good arch and is very strong. Although tone took a hit, going away from the ramp and allowing the nail to grow out well past the nail bed has been the trick for me.
If you look at extreme examples of people who grow their nails very long, the nails will tend to grow out to a point and then start to spiral. Allowing a good amount of white on each edge of the nail and using a more rounded shape has completely changed the shape and strength of my nails.
It may have sounded slightly unsafe and barbaric when a previous poster advocated pushing the nail bed back at the finger tip to get a good shape and expose more white. I do think however that they were on to something. The drier white area on the nail at each side, at least for me, has been responsible for causing the nail to curve inward, eliminating the hook entirely and giving the nail strength.
1973 Ryoji Matsuoka No. 30 Cedar 650mm
1970s Fana 300L Cedar 630mm
2016 Benito Huipe Clasica Cedar 655mm
1972 Yamaha GC-6D Ezo Spruce 662mm