Hi! Not at computer right now, but a common mistake is UVs not being split at hard edges. Hard edges are useful at steep angles to control the lowpolys shading - a smooth curvature wouldn't qualify for hard edges (but can still have a UVs split). Your post is missing images of the lowpolys shading and UV layout, so hard to…
UVs and shading should work. No hard edges, so what I wrote before doesn't apply here. With the lowpoly being constructed out of individual boards, I would try using Bake Groups (if you don't already) to prevent adjacent geometry being projected into the normal map. Highpoly seems pretty heavy and could be decimated it a…