While all those visual enhancements do look cool, would it be a big problem to have an option in the GUI that would allow the user to enable/disable markdown visual styling at will? I'm talking about something done with a simple tick, and not having to rely on custom CSS to achieve.
I myself prefer to have markdown displayed as if it was plain text, and I know I'm not alone (see https://discourse.joplinapp.org/t/is-it-possible-to-get-the-markdown-editor-to-behave-like-a-plain-text-editor/20337). For me, one of the main reasons is that I usually find the default colours having too low contrast, and as such being difficult to read, especially anything grey on white background. Personally, I'd even consider this an accessibility issue.
I'm also not a fan of the gigantic # heading text, but this is more of a personal preference.