This seems to be a very nicely triangulated surface. To make triangles less elongated, you can resample your input volume to have cubic shaped voxels, by checking Isotropic spacing checkbox in Crop volume module when you crop/resample your original volume to prepare it for segmentation.
If you think they are better than the one in slicer, it would be a good idea to integrate this work in slicer. I am planning to integrate them as a slicer extension in the upcoming weeks (if someone is not done this before).
Triangle size is determined by the input volume’s resolution. You can decrease the number of triangles in flat regions by replacing them by larger ones by using decimation option (you specify the fraction of points that should be removed, so decimation factor of 0.9 means that you remove 90% of the triangles).
Segmentation generates a surface mesh. To generate volumetric mesh, you can use SegmentMesher extension. SegmentMesher uses Cleaver2 or Tetgen meshers internally, which are well-established algorithms.
instant-meshes and robust_hex_dominant_meshing methods seem to be for remeshing of existing surface mesh to improve mesh quality. I don’t think they are relevant for us, as from images we can always generate high-quality meshes that contains triangles with equal size with a nice aspect ratio.