Hi there 😊,
Yes, an import script would be the right place to reduce manual steps.
I presume if you create a tool script which does the initial split of an AnimationPlayer into multiple ones you can use it for importing and changing already imported resources. (E.g. clicking the node and have a new popup to "split this" as an editor script).
I recommend taking a look at AnimationMixer, which contains the AnimationLibrary you want to split and is the base class of AnimationPlayer. Not sure if duplicate() works out-of-the-box for all involved components or if you have to duplicate the involved parts yourself via code.
https://docs.godotengine.org/en/latest/classes/class_animationmixer.html#class-animationmixer-method-get-animation-library
May I ask why you want to split animations on their own player each? I presume this might lead to concurrency issues if the players are not synced correctly in usage and frankly, though I think it should be possible to do this, I have no knowledge if this actually has been done or is supported.