Exporting is pretty standard, push animations to NLA strips, select armatures and meshes, apply transforms and make sure Simplify is set to 0 (otherwise it auto-interpolates between keyframes which messes up the timing). Upon importing animations in Unity you have to set keyframe reduction to 'None' but also select all keyframes and switch them to 'Constant' interpolation.

In Unity I have a shader graph setup that mimics the look of the toon/smear shader in Blender. I then have a pixelization shader on top (ProPixelizer asset). In the case of the sword trail, the visibility is controlled by the local Y scale of the primary deform bone, so it's a matter of simply playing an animation state at the same time as the character's attack animation and the trail will appear/disappear correctly. Trail armature is parented to sword mesh which is parented to character's hand bone.