You shouldn't allow your personal preference to guide you which forum software to use. It's quite obvious when you visit tons of forums that millions of people are quite happy without having a list of topics on one side.

"The biggest one is since phpBB is so popular it is a frequent target for automated attacks by spammers"

I don't think you have informed yourself all that well. PHPBB might be a frequent target, but that doesn't mean that it suffers from these attacks with the latest updates. And no, I'm not running PHPBB on my two sites, so that's not the reason why I'm saying this. I use SMF and I"m very satisfied with it, no problems with spammers ot hackers whatsoever.

And if you really want to use a topic column on the side, then make one. In SMF this is not even difficult to do, because of built in extensions/modules that are easy to use.

What you now have is out of date, difficult to use and most important not very helpful, since things are not well organized. This is something that shouldn't be fixed after you release the product, you should have done this from the start. Now you're stuck with tons of old posts that are probably difficult to convert once you decide to use other board software. So basically you will continue to struggle with this forum.

Don't underestimate the importance of a good forum, because that's what I'm sensing right now. Get rid of this, excuse me for saying this, junk and get yourself something that is more useful for your future customers, they will appreciate that a lot, trust me on that one.