Tip (a lot of people don't know this): you can set the resolution by setting the $fn variable. This can be useful when you want to export smooth objects.

  $fn = $preview ? 32 : 64;

Just a minor aside: When exporting geometry for FEM, you generally should not make $fn very high (values around 32–64 are usually fine). High $fn improves visual smoothness, but it often hurts FEM meshing and solver performance without improving accuracy, since FEM accuracy is governed by the solver’s mesh rather than the CAD tessellation.