Dynamesh requires meshes with thickness.
ZRemesher can work on meshes without thickness, but yours is a hard surface mesh which has special concerns. Generally speaking, Zbrush works best on meshes that are closed volumes with a degree of thickness. You should always aim for meshes in this form if you want to work on them in Zbrush. It would make your job easier to extrude some thickness for your mesh, and delete the unwanted geometry prior to export.
There is no auto-retopology magic button in any software Im aware of to create optimal low poly hard surface topology of the quality a human would produce if modeling from low poly. It’s a process, a developed skill, and some cases may require manual retopology.
Hard surface models are further complicated by the fact that low poly models require creasing in order to keep edges hard when being smoothed during subdivision. If no smoothed or curved surfaces are required( the mesh is all flat planes), then smoothing can be disabled during subdivision by disabling the SMT button next to the Divide button in the Geometry Palette.
However, ZRemesher can produce good results if your mesh shape isn’t overly complex. Unfortunately your mesh is fairly complex and has subtle elevations and fine details that aren’t likely to survive the ZRemesher process no matter what settings you use. These areas may need to be manually retopolgized with ZModeler and/or the topology brush, or reconstructed entirely on the retopo’d mesh.
I find it’s best to pick one strategy for doing this, and not to activate multiple edge detection options at once (for instance, keep groups, detect edges, or keep creases, but not all at once) . Ticking multiple options may conflict with one another, and will increase the performance burden of ZRemesher, and the amount of memory and time required to complete.
I get the best result by first making sure every surface that is to be separated from another by a hard edge is a separate polygroup, and using the “Keep Groups” option. Then ZRemesher will do a much better job of drawing edges between the polygroup borders. It also has the benefit of allowing you to quickly establish creasing between the polygroup borders afterward with Tool> Geometry> Creases> Crease PG.
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Don’t chase the perfect results with ZRemesher. Get in the ballpark of what you need, then rewire problem areas with ZModeler or Topology brush.
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ZRemesher may understand your mesh better if you subdivide it, giving it more points on the surface to evaluate. But remember about creasing.
