Try to be more conservative with your thresholds, make sure the areas that you include in a segment are definitely correct. Then use Grow from seeds effect to create complete segmentation from those “seed” regions.
You can also use Watershed effect (available in SegmentEditorExtraEffects extension), which takes the same input as Grow from seeds but produces a smoother output. Smoothness is controlled by Object scale, try with 1mm and adjust as needed. The main disadvantage of Watershed is that it is slower compared to Grow from seeds.