EditorGUI, EditorGUILayout, GUI, GUILayout... pshhh... WHEN TO USE WHAT?!

EditorGUI.Toggle and GUI.Toggle looks different but I hear your frustration as I've been working with Editor GUI code a lot and know where you're coming from. Look at this example for example, which does a toggle with EditorGUILayout and GUILayout. Not only are the parameters swapped around but the output looks different too. EditorGUILayout.Toggle is used in inspectors to get a uniform "Label" - "Content" structure just like the (well, most, at least) other EditorGUI methods.

 using UnityEngine;
 using UnityEditor;
 
 public class MyEditor : EditorWindow {
     [MenuItem("Example/Show Window")]
     public static void ShowWindow () {
         GetWindow<MyEditor> ().Show ();
     }
 
     void OnGUI() {
         EditorGUILayout.Toggle ("EditorGUILayout.Toggle", false);
         GUILayout.Toggle (false, "GUILayout.Toggle");
     }
 }

I think the best advice I can give you is to get a feel for the different classes and accept that you may have to jump around in them to find what you need. Don't be afraid of mixing class usage. In the case of BeginDisabledGroup, it has nothing to do with laying out of elements, so I guess that's why it ended up in EditorGUI rather than EditorGUILayout.

It is what it is. We have to accept it and it will add a little learning curve to us all, but once we got warm with it, it's relatively easy to get around and being productive.

Per the discussion in the comments, here's how you can encapsulate your GUI code into reusable classes that hopefully are easier to manage.

The idea is that you should only worry about how complex the first class looks. The IGUI interface is just for convenience here, to allow all GUI objects to be added to a list. I include a button and a text field. As you can see, the editor only has one variable to each GUI control, username, realname and registerButton. They are created during OnEnable, and a callback is set up so button clicks will be caught by RegisterUser. RegisterUser will just demonstrate that it pulls data from the two text fields.

This is a pretty minimal example - you can flavour it in any way you want. Perhaps you want to expose styles to your objects too for instance. Perhaps you want to be able to disable individual components? And so on.

 using UnityEngine;
 using UnityEditor;
 using System.Collections.Generic;
 
 public class MyEditor : EditorWindow {
 
     GUITextField username;
     GUITextField realname;
     GUIButton registerButton;
 
     // Optional, but may be convenient.
     List<IGUI> gui = new List<IGUI>();
 
     [MenuItem("Example/Show Window")]
     public static void ShowWindow () {
         GetWindow<MyEditor> ().Show ();
     }
 
     void OnEnable() {
         username = new GUITextField ();
         username.label.text = "Username";
         username.text = "JDoe";
 
         realname = new GUITextField ();
         realname.label.text = "Real name";
         realname.text = "John Doe";
 
         registerButton = new GUIButton ();
         registerButton.label.text = "Register";
         registerButton.Clicked += RegisterUser;
 
         gui.Add (username);
         gui.Add (realname);
         gui.Add (registerButton);
     }
 
     void RegisterUser()
     {
         var msg = "Registering " + realname.text + " as " + username.text; 
         Debug.Log (msg);
     }
 
     void OnGUI() {
         foreach (var item in gui)
             item.OnGUI();
     }
 }
 
 public interface IGUI {
     void OnGUI();
 }
 
 public class GUITextField : IGUI {
     public string text = "";
     public GUIContent label = new GUIContent();
 
     // Unused in my example, but you may want to check if
     // a textbox becomes empty for example.
     public event System.Action<string> TextChanged;
 
     public void OnGUI() {
         // Also I wanted to show you BeginChangeCheck and EndChangeCheck 
         // which is the Unity GUI way of checking if a GUI control changed...
         EditorGUI.BeginChangeCheck ();
         text = EditorGUILayout.TextField (label, text);
         if (EditorGUI.EndChangeCheck () && TextChanged != null)
             TextChanged (text);
     }
 }
 
 public class GUIButton : IGUI {
     public GUIContent label = new GUIContent();
     public event System.Action Clicked;
 
     public void OnGUI() {
         if (GUILayout.Button (label) && Clicked != null)
             Clicked ();
     }
 }