I am considering using Mautic (for a client). Our client has some unique requirements. The biggest needs are as follows:

Be able to use a WYSIWYG builder to add assets to an existing email template. In other words, the client would like the entire email to essentially be locked down to most marketing users. The header, footer and other images should be static based on an original design. The only thing that the marketing user would be able to do is change the body copy and some other elements. Even the way that the CSS is set up will need to be locked down.

They are currently using a custom solution, but every email that they send needs to go through a development team, and they are trying to avoid that. Because of their unique requirements they have created their custom solution, but they would like to be able to add emails without the use of developers, so I thought that an open source solution might be best.

Would appreciate your thoughts on how this might be able to be accomplished (or if it can be accomplished at all). Thank you all in advance.