you could PBR render to preview with smooth normals. but honestly for the best quality possible, just do not decimate. unless the meshes are ridiculously dense i would just bake with the highest geo possible, cuts out any errors that might come from decimation.
Personal project. Working from a Tyson Murphy concept https://www.artstation.com/artwork/bOVgG was thinking of hand painting this (similar to this https://www.artstation.com/artwork/4X2rY), but i think i'll make it PBR.