If you are talking about this plugin https://wordpress.org/plugins/easy-table-of-contents/ it says that there are filters and hooks, but I don’t see any information on them. I would talk to them. If it’s another plugin then you can try contacting the author.
There are ways to insert ACF fields into the content, but that would really mean that you’d hide the content editor, then create an acf/save_post filter that would parse all the fields, created all the needed HTML and then update the post with the new content. Likely a complex project, but possible.
