The result still has a white rectangle
Lower Background removal slightly and check the result on a dark background. Export PNG, not JPG, because JPG cannot preserve transparent pixels.
Thin strokes look faint
Move the setting left to preserve lighter ink. A sharper source scan with even light usually gives the biggest improvement.
The signature has large invisible margins
Keep automatic trimming enabled and crop the source roughly around one signature before processing.
Blue ink or notebook lines remain
Use Original or Blue for ink color and crop out lines that touch the handwriting. Preview on both light and dark backgrounds.