Unfortunately I'm not able to spend time on this one anymore. I cannot give any concrete advices either not knowing what is your stack; if I knew it there is a high chance I wouldn't be able to anyway.
A non-concrete advice then: you could try to do something like storing positions and rotations (or just transformations) of the weapon from the most resent 8 or so frames; than draw it with a diminishing opacity. Draw the actual sword the last (on top of everything else).
Maybe instead of opacity you should always draw them with a shader and a plain color of choice (the color of the weapon or white or some fantasy red/blue whatever) - maybe adding glow shader to the mix would help.
Finally you can use particle to draw sword trails; from my experience particles in pixel-art looks great if they obej pixel-artness of the game : ) - there are some tutorials available out there, mostly for 3d, but it shouldn't be a blocker.
Good luck with your project!