No screenshots attached, so it’s hard to interpret.
Depending on what animation set you are using, issues with bad bone translation on the wrist may be part of the original animation.
exporting the original mannequin animation and passing it through BoneBreaker usually resolves this (it used to before 2.82 at least).
Within the retarget manager, change all your bones to use the Animation translation to see whatever the original animation detects may be.
Because of the default settings those defects usually are not visible within ue4.
As far as difference between frames. It may be normal if it isn’t overly exaggerated like returning to the A pose.
a proper animation cycle has the same animation at the start and end, but is then cut short by 1 frame so that it can loop flawlessly.
If you are trying to replicate that, you can then just copy the first frame to after the last one.