UNITY sprite auto slicer. Put into any Editor/ directory. Use it in combination with Presets/PresetManager for importer settings.

AutoSpriteSlicer.cs

using System.Collections.Generic;
using System.IO;
using System.Linq;
using UnityEditor;
using UnityEditorInternal;
using UnityEngine;

public class AutoSpriteSlicer : AssetPostprocessor
{
    private const string PathTrigger = "_AnimationSheet";
    private const char FileNameSeparator = '_';
    private const char SizeSeparator = 'x';
    
    private static string _processedFile;
    private void OnPostprocessTexture(Texture2D texture)
    {
        if (!assetPath.Contains(PathTrigger)) return;
        if (_processedFile != null)
        {
            _processedFile = null;
            return;
        }
        
        _processedFile = assetPath;

        TextureImporter textureImporter = (TextureImporter)assetImporter;

        string fileName = Path.GetFileNameWithoutExtension(assetPath);
        string[] rectangleSize = fileName.Split(FileNameSeparator).Last().Split(SizeSeparator);

        if (rectangleSize.Length < 2) return;

        Vector2 spriteBoxSize = new Vector2(int.Parse(rectangleSize[0]), int.Parse(rectangleSize[1]));

        Rect[] rectangles = InternalSpriteUtility.GenerateGridSpriteRectangles(
            texture, 
            Vector2.zero, 
            spriteBoxSize, 
            Vector2.zero, 
            true
        );

        int rectNum = 0;

        textureImporter.spritesheet = rectangles.Select(rect => new SpriteMetaData
        {
            pivot = Vector2.down, 
            alignment = (int) SpriteAlignment.BottomCenter,
            rect = rect, 
            name = fileName + "_" + rectNum++
        }).ToArray();
        
        AssetDatabase.ForceReserializeAssets(new List<string> { assetPath });
        AssetDatabase.ImportAsset(assetPath, ImportAssetOptions.ForceUpdate);
    }
}