Instant Meshes (free) is your best bet — import your messy FreeCAD mesh and it'll auto-generate clean quad-dominant topology based on surface curvature. For architectural geometry specifically, try Decimate (Planar mode) + Limited Dissolve in Blender first to merge coplanar faces before retopologizing — cuts down a lot of the chaos for free. RetopoFlow or Quad Remesher (paid) give more control if you need it after that.