Not sure how the font option exactly works. I like to write using a monospace but it also change how the post is displayed to readers. is there an option to dissociate both? I switch to a serif font when publishing but that’s extra-friction.
I looked into the documentation but did’nt find mention of this. Will be glad to contribute when I am sure about this 
Yep the font selection is specifically for how it shows up on your blog for other readers.
I don’t know that we’ll want to change this in the future, so I’d recommend using something like a desktop text editor if you like to see a different font than what you’re publishing with.
Not looking to change your mind on this and only sharing a personal opinion
font selection should be at the blog level and not on the article. This will reduce both look and feel (since all the articles of one blog will have an aligned appearance) and cognitive load (since you remove a decision for each time you want to publish an article). It will allow to focus on writing, editing and publishing.
Regarding, writing, I really appreciate the simplicity of WF ui. This reminds me a lot of iAWriter because both are opiniated. It will add an extra-burden to have another software layer in my workflow. I was previously using neovim/iawriter + git + 11ty previously and I am happily moving away for something more streamlined. Having my drafts directly in my CMS instead of a bunch of files PR I have to manage is a real win to me.