Create First Person Hands Animation Speedrun
The general idea is to use another animation set(Low Poly FPS from Asset Store) to drive my own character's first person hands to animate.
So with hands IK to match it, modify the fingers a little bit, use another Animator to animate the bones so IK won't look too weird, Then bake the animation into AnimationClip. If there are still wrong results, adjust it with UMotion