To be honest, I will never use a WYSIWYG editor that didn't show the original code......
SimpleMDE is I think the best for flarum, it's light, you have a fullscreen function like flarum, you can see markdown, you have 2 ways to visualise code (one is a fullscreen splitting screen ‼ ), it is phone friendly too...
Brief... It depend a lot of the user in fact... But that my taste !!
SimpleMDE (or similar) must be a core feature and then have a choice of extension for the user side that completely hide the markdown.. But I always say that it's a mistake, markdown is really easy and if random people don't know it, it's just cause webmaster always put a freaking WYSIWYG editor !!
Hahahahaha I just see that on quill they say "built for developers".... hahaha the joke !!
A dev need to see the code.... (Or my life is a lie) ? "Quill, built for developers who don't give a shit about markdown !!" hahahaha ?