Looking for a web WYSIWYM editor

I am looking for a WYSIWYM web editor to integrate with a commercial web application (using TYPO3) we are developing.

Short version: We need a web editor where we can add custom widgets that will be rendered only once the HTML is generated, and we want the editor to be as much user-friendly as possible.

Long version: We need an editor for letting a user to write email templates in our application. Basic formatting are OK for us, but users need to be able to insert conditional boxes (to make e.g. greetings conditional to the recipient gender), attributes (variables such as recipient name, number of booked tickets) and custom elements that will be rendered when sending the email (such as booking details, recipient address). We would like to have these conditional boxes, attributes and custom elements diplayed in the editor as something like a box (+/- like a ERT in Lyx). User should also be able to insert raw HTML code in a code box (like an ERT in Lyx).

ProseMirror can probably fit our needs, but it has a very steep learning curve. My question is whether there are user-friendly WYSIWYM alternatives. I am wondering if Quill, Trix or Slate can do it.

Thanks in advance for your answers, Jonas