Example of how to add upgrade levels to Atos Arrows XBow without a framework.

Main.cs

using BepInEx;
using HarmonyLib;
using JetBrains.Annotations;
using System.Linq;
using System.Reflection;
using UnityEngine.SceneManagement;

namespace Digitalroot.Valheim.AtosArrows.Crafting.Recipes
{
  /// <summary>
  /// Example of how to add upgrade levels to Atos Arrows XBow.
  /// This example does not require any frameworks.
  ///
  /// Open Visual Studio and create a new Class Library. Target .Net Framework 4.6.2
  /// Add the nuget package 'Pfhoenix.Valheim.ModProjectReferences' to your project.
  ///
  /// Replace the code in the Class1.cs file with the code here.
  /// Build the project or solution.
  /// Copy the output DLL into the BepInEx plugins directory. Look in [project directory]\bin\Debug
  /// e.g. my DLL is named Digitalroot.Valheim.AtosArrows.Crafting.Recipes.dll.
  /// You only need to copy your DLL and nothing else from [project directory]\bin\Debug
  /// </summary>
  [BepInPlugin(Guid, Name, Version)]
  [BepInDependency("com.bepinex.plugins.atosarrows", "0.6.0")]
  public class Main : BaseUnityPlugin
  {
    public const string Version = "1.0.0";
    public const string Name = "Digitalroot Atos Arrow Crafting Recipes";
    public const string Guid = "digitalroot.mods.AtosArrows.Crafting.Recipes";
    public const string Namespace = "Digitalroot.Valheim.AtosArrows.Crafting.Recipes";
    private Harmony _harmony;

    [UsedImplicitly]
    private void Awake()
    {
      _harmony = Harmony.CreateAndPatchAll(Assembly.GetExecutingAssembly(), Guid);
    }

    [UsedImplicitly]
    private void OnDestroy()
    {
      _harmony?.UnpatchSelf();
    }
  }
  
  public class Patch
  {
    [HarmonyPatch(typeof(ObjectDB), "Awake")]
    public class PatchObjectDBAwake
    {
      [UsedImplicitly]
      [HarmonyPostfix]
      [HarmonyPriority(Priority.Normal)]
      [HarmonyAfter("com.bepinex.plugins.jotunnlib", "com.bepinex.plugins.atosarrows")]
      public static void Postfix()
      {
        ZLog.Log($"{Main.Namespace}.{MethodBase.GetCurrentMethod().DeclaringType?.Name}.{MethodBase.GetCurrentMethod().Name}");
        FixXBow();
      }
    }

    private static void FixXBow()
    {
      ZLog.Log("Updating Atos Arrow Recipes");

      // This is normal checking if ObjectDB is ready. 
      if (!(ObjectDB.instance != null && ObjectDB.instance.m_items.Count != 0 && ObjectDB.instance.GetItemPrefab("Amber") != null))
      {
        ZLog.Log("ObjectDBReady not ready - skipping");
        return;
      }

      // com.bepinex.plugins.atosarrows waits to load until the Active Scene is 'main'. Most other item mods load on Awake and CopyOtherDB
      if (SceneManager.GetActiveScene().name != "main")
      {
        ZLog.Log("Not at 'main' Scene - skipping");
        return;
      }

      // We need to know the ItemId of the item we want to change. For the Crossbow the ItemId is 'XBow'.
      // This foreach should return any Recipe that results in creating a XBow. This should solve an 
      // edge case where an item has more then one Recipe to craft it.
      foreach (Recipe instanceMRecipe in ObjectDB.instance.m_recipes.Where(r => r.m_item?.name == "XBow"))
      {
        instanceMRecipe.m_item.m_itemData.m_shared.m_maxQuality = 3; // Sets the max level an item can be upgraded to.
        ZLog.Log($"Updated {instanceMRecipe.m_item.name} of {instanceMRecipe.name}, set m_maxQuality to {instanceMRecipe.m_item.m_itemData.m_shared.m_maxQuality}");

        // com.bepinex.plugins.atosarrows did not define m_amountPerLevel values. Without these the item is upgradable for free.
        // The original Recipe uses Crystal, BlackMetal, FineWood and LinenThread. For the upgrade only more Crystal, and BlackMetal
        // are required. However more FineWood and LinenThread could also be required.
        foreach (Piece.Requirement requirement in instanceMRecipe.m_resources)
        {
          switch (requirement.m_resItem.name)
          {
            case "Crystal":
              requirement.m_amountPerLevel = 5;
              break;

            case "BlackMetal":
              requirement.m_amountPerLevel = 10;
              break;
          }
        }
      }
    }
  }
}