Is there any way to toggle a hide/display setting that would let the user customize the main forum index page in order to streamline it by not showing boards that aren't relevant to him/her?

Try this. It might help. On the index page just under the logo it says,Welcome to the Magic Café. Click on that or any other main forum heading and you'll see that all of the subtitles have gone away and only main forums are listed. Say you are interested in money magic. Click on "show me the money" and that forum will open with all the sub titles. When you're finished, click on the logo and the index will return to normal.