How to change spine character's weapon and clothes efficiently in Godot？
I think you can use skins.
Exactly, skins are the preferred way to do this. See the mix-and-match example of the spine-godot runtime and the editor documentation here:
By the way，is there any similar API to GetRepackedSkin(spine-unity runtime API) in Godot C#？
There is currently not for technical reasons.