I don't really think it's possible, even though we can like SMF for the structure it brings on other parts, I believe it is very limited on the "unread" and "recent" part of the forum. It's probably really just how this very forum works in general, and I'm not certain if there's any mods/addons that you can install that modifies it in such a manner as suggested here.
The "show unread posts since last visit" is really nothing at all as long as it's not clicked. The unread posts is throughout the forum since your recent visit whether it was posted in or not, newest thread and post ontop older the further down, it's there as a general forum function for those who want to just see the newest content.
The option below is for you and your threads you have replied or posted in. It will only show the threads that you actually replied to and nothing else, newest thread on top here as well.
I can admit, and other users has been pointing out this before, that once you have posted in a thread once, and people keep posting in it, you will not be able to "unsubscribe" from this in the "Show new replies to your posts" which would be handy, but sadly that's how it works.
With that as an exception however, the rest is fairly easy to understand and get by, and bear in mind that the first option "Show unread posts since last visit" is not personal, the same content is displayed for everyone since it's about the newest threads and posts on the board. If you want a personal list you check the "Show new replies".
Hope this helps out.