My colleague mentioned about the simplified version but in my opinion, at last, it will not be that much easy as I see you expect to have a simple place to only add TEXT.
But if you used the Off Canvas and added the HTML code there, unfortunately, that will be a dead end for your client as there will be no way to simplify that even with the upcoming update.
The one step we took regarding ease of use is the Template Manager and GLobal Blocks. But I still am not convinced that will make completely nontechnical guys work easier.
If you want to have more complex stuff easier you will need to use ACF Pro.
The idea is that you add the proper elements which will be easy in the admin for example to change the promotion message. Then you need to code that to the front end of the website. You can use shortcodes of the ACF Pro to add to the Off Canvas:
That way you can have the complex HTML in the Off Canvas and only have the text part feeding on ACF Pro which as a simple place for your customer.
The plugin is bundled with the theme but it has a steep learning curve. Luckily they have a great documentation.