CreateSymlinkProject.cs

#if UNITY_EDITOR_WIN
using System;
using UnityEngine;
using UnityEditor;
using System.Diagnostics;

class CreateSymlinkProject {
	[MenuItem("Tools/Create Symlink Project")]
	static void SymlinkProject() {
		var src = Application.dataPath.Replace("/Assets", string.Empty);
		var dst = EditorUtility.SaveFolderPanel("Create new project root folder", string.Empty, string.Empty);
		if (dst == string.Empty)
			return;

		var cmdBatch = new[] {
			Symlink(src, dst, s => $"{s}/Assets"),
			Symlink(src, dst, s => $"{s}/ProjectSettings"),
			Symlink(src, dst, s => $"{s}/Packages"),
			Copy(src, dst, s => $"{s}/Library"),
			Copy(src, dst, s => $"{s}/Temp")
		};

		foreach (var cmd in cmdBatch) 
			CmdAdmin(cmd);

		Application.OpenURL(dst);
	}

	static string Symlink(string src, string dst, Func<string, string> local) {
		return $"/c mklink /D \"{local(dst)}\" \"{local(src)}\"";
	}

	static string Copy(string src, string dst, Func<string, string> local) {
		return $"/c robocopy \"{local(src)}\" \"{local(dst)}\" /MIR /R:0 /W:0";
	}

	static void CmdAdmin(string command) {
		// elevate permissions to admin
		var processStartInfo = new ProcessStartInfo("cmd.exe", command) {
			// CreateNoWindow = true,
			// WindowStyle = ProcessWindowStyle.Hidden,
			UseShellExecute = true,
			Verb = "runas"
		};

		Process.Start(processStartInfo).WaitForExit(); // waiting prevents Library folder modifications while copying
	}
}
#endif