Does the lowpoly shading look good with the normal map applied? Hard edges are certainly useful to control the shading of the lowpoly, especially at steep angles - just remember to split the UVs along hard edges to avoid artifacts! Maybe check out Making sense of hard edges, uvs, normal maps and vertex counts?
comes down to the angles on the surface, the normalmap might just carry it.Tho even with a working normalmap, we could get issues once the map is compressed, or mipped the problem arent the diagonals, those are there whether you make em visible or not. the issue is the undercut behind the gill what you could also do, is…