Before we proceed, have you considered enabling the Append Changes to Existing Text option in SharePoint?
SharePoint list apps offer an append feature for columns of the type Multiple lines of text. When you enable this feature, changes are shown as a thread in the field. The earlier text remains uneditable, while new text is added above the previous content. This feature is especially useful for discussions and issue tracking, as it provides a clear history of updates.
By enabling this feature, you wouldn't need to worry about implementing the append logic manually.