Hi Meer,
When you use a "mailto" anchor, you are just instructing your mobile browser to start your mobile e-mail app. That's indeed fairly limited in what you can achieve. Also, you don't actually send any e-mail in that case, you just fire up your e-mail app. If that's what you want, you could take a look at this plugin.
As for the Server Action, if you want to pass the HTML content, use an Input Parameter to the E-mail Screen (if there's not too much HTML), or store it in the database and use the Id as Input to the Screen (and retrieve the content with an Aggregate in the Preperation). Next have an Expression in the E-mail Screen that you populate with the HTML (set Escape Content to False). Make sure the E-mail Screen is in it's own Flow, using it's own Theme (or you'll get a lot of CSS that you don't want sent along with it).