Calling a method on a serialized property (Custom Editor with Reorderable List)

I am trying to create a tool that spawns maps randomly on given propabilities for tile neighbors. Therefore I have written classes for a Tile and a Propability:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 [System.Serializable]
 public class Tile
 {
     public string description;
     public Sprite sprite;
     public List<Propabilities> propabilities;
 
     public void AddPropability(Tile neighbor)
     {
         propabilities.Add(new Propabilities(neighbor));
     }
 
     public void RemovePropability(Tile neighbor)
     {
         propabilities.Remove(propabilities.Find(x => x.neighbor == neighbor));
     }
 }
 
 [System.Serializable]
 public class Propabilities
 {
     public Tile neighbor;
     public float propability;
 
     public Propabilities(Tile neighbor_)
     {
         neighbor = neighbor_;
         propability = 0;
     }
 }

The MonoBehavior Class 'MapManager' currently only contains a List of Tiles. I have written a Custom Editor for the MapManager and for the Tile class using Reorderable Lists for the List of Tiles and the List of Propabilities in each Tile. Therefore I read into a lot of examples using Serialized Objects and Properties which still tend to confuse me, however I got along quite well until this point. I now have to update each Tile's List of propabilities every time a new Tile is added to the Editor. I tried to use casts, which do work on Sprites for example:

 SerializedProperty sprite = element.FindPropertyRelative("sprite");
             EditorGUI.PropertyField(
                     new Rect(rect.x, rect.y + EditorGUIUtility.singleLineHeight + 2 * gap, headerWidth, EditorGUIUtility.singleLineHeight),
                     element.FindPropertyRelative("sprite"), GUIContent.none);
 
             if (sprite != null && sprite.objectReferenceValue != null)
             {
                 EditorGUI.DrawPreviewTexture(new Rect(rect.x, rect.y + 2 * EditorGUIUtility.singleLineHeight + 3 * gap, headerWidth, headerWidth),
                     (sprite.objectReferenceValue as Sprite).texture);
             }

However, this doesn't seem to work on own classes. How do I get access to the Methods in my class?

I'm still not sure if I completely understand what I'm doing here, so if there's a more simple approach to what I'm trying to do, I'm open for ideas.

Any help would be much appreciated. Thanks :)