Just to clarify, we used the tools and plugin template in this Drupal module to get started:
https://www.drupal.org/project/ckeditor5_dev →
I offer one solution:
We were able to solve this by creating a custom CKE5 plugin in a Drupal custom module. The plugin registers the new model element, creates the conversions for the custom html tag for the view, creates the command to apply it, and assigns it to a button in the editor toolbar. It took a learning curve through CKE5 documentation (https://ckeditor.com/docs/ckeditor5/latest/api/index.html), but the positive of that is our new clarity on CKE5 conception and architecture.
Creating this plugin registered the custom element and the custom html tag. It is now in the "Allowed HTML tags list" and is no longer filtered out.
Thank you Wim Leers, Lauriii, and all for the cool work on the integration!
After I submitted this issue, Email TFA v1.0.6 was released in which another issue already solved this much better than my proposed patch! Thank you to the maintainers, and to mariacha1.
Please close this issue as duplicate but solved.