Getting a managed reference out of a SerializedProperty is very challenging¹⁾. It's better to just use C# reflection on the original object and force the asset / object to re-save.

See:

• https://forum.unity.com/threads/setting-a-serializedproperty-to-a-struct.521752/
• https://answers.unity.com/questions/1347203/a-smarter-way-to-get-the-type-of-serializedpropert.html ²⁾