This post from @laurent gives details of the latest Joplin Pre-Release which has (amongst other things) an alternative Markdown editor.
New Markdown editor
A new Markdown editor (Code Mirror) is now included with Joplin. For now it is for beta testing only so you need to manually enable it in the options in “Note” => “Use CodeMirror as the code editor”. The goal of this new editor is to address several of the issues that have affected the previous Markdown editor and that were either hard or impossible to fix, in particular:
Support for spell checking. Note that it is not yet implemented in the new editor but it at least can be done now.
Variable font size. The previous editor would not allow this, which would cause various issues.
Search within the Markdown text. Previously it was only possible to searched with the note viewer but with this editor it should be possible to search within the Markdown text too.
Improved support for emojis.
Improved support for Asian languages and Russian.
I have only briefly looked at it but it seems to do some of the things you mention.