Once I use the split or trim tool it leaves a deformation line.even in high res see
I think on the top inages that the problem is that the trim line crosses the symmetry line. You’re seeing a doubling up of the trimmed mesh on either side of the line & it’s messy polys. It doesn’t always happen but when it does, I simply turn off symmetry, trim one side & then mirror.
On the bottom images, not sure what your split settings are for hole closing. A bit of experimentation and you’ll find the sweet spot for detail / smoothing & threshold.