Adjusting start and end frames of animation clip with script

I have a .fbx model which is imported into Unity. (version 2019.3)
There is one animation clip attached to the model.

I want to cut the animation clip into multiple clips based on some separation data that I have (the start and end frames of each piece). I want to do this automatically and the process which I would follow manually goes like this:
* Click on the model file. On the inspector, go under the Animation tab and modify the start and end frames of the animation.
* Click on APPLY
* Expand the fbx file on the project window. Then duplicate the animation clip file. That's one clip.

I want to automate this procedure. I found that I don't have access to a clip's start and end frame. So I tried to use ModelImporter to access the import settings of the model and mimic the procedure above. Using that, I tried to also use ModelImporterClipAnimation and then change the fields firstFrame and lastFrame. Then, I use Instantiate() to copy the newly modified animation clip and save that copied clip somewhere else.

My problem is that the clip doesn't seem to be modified. When I use Instantiate(), the whole clip is copied without any change to its start and end frames. I tried using SerializedObject as an attempt to mimic the pressing of the APPLY button in the model settings window, but I had no success.

Any ideas about how to simplify the process or of what I am doing wrong?

 

Code (CSharp):

1. private void SplitAnimation(string file, int splitFramePoint)
2. {
3.     var fileName = Path.GetFileNameWithoutExtension(file);
4.     var fileNameParts = fileName.Split('_');
5.     fileNameParts[2] = (int.Parse(fileNameParts[2]) + 1).ToString("D2");
6.     var newFile = file.Replace(fileName, string.Join("_", fileNameParts));
7.     AssetDatabase.CopyAsset(file, newFile);
8.                                                                                        
9.     {
10.         var animationImporter = AssetImporter.GetAtPath(file) as ModelImporter;
11.         var defaultClipAnimations = animationImporter.defaultClipAnimations;
12.         foreach (var animationImporterClipAnimation in defaultClipAnimations)
13.         {
14.             animationImporterClipAnimation.lastFrame = splitFramePoint;
15.         }
16.         animationImporter.clipAnimations = defaultClipAnimations;
17.         animationImporter.SaveAndReimport();
18.     }
19.                                                                                        
20.     {
21.         var copiedAnimationImporter = AssetImporter.GetAtPath(newFile) as ModelImporter;
22.         var defaultClipAnimations = copiedAnimationImporter.defaultClipAnimations;
23.         foreach (var animationImporterClipAnimation in defaultClipAnimations)
24.         {
25.             animationImporterClipAnimation.firstFrame = splitFramePoint;
26.             animationImporterClipAnimation.lastFrame -= splitFramePoint;
27.         }
28.         copiedAnimationImporter.clipAnimations = defaultClipAnimations;
29.         copiedAnimationImporter.SaveAndReimport();
30.     }
31. }