Hi! Regarding the lowpoly, I would partially collapse some edge loops, have them split as the shape gets larger or in areas where more geometry is needed for silhouette relevant details - make use of triangles. Design wise, the shapes lack a bit of flow and dynamism imo. With the initial concept being rather vage/flat, no…
The main thing to remember in retopology is to maintain your silhouette. You have a bunch of loops in the back of the statue that aren't needed and could be collapsed into triangles. Something like this but you could definitely take it further