I mean if you literally want to see North Korea, by all means do a Non-JSA tour. That being said, the Non-JSA tour is not the best because it just looks like the regular Korean countryside over there. If you want the feeling of SUPER SCARY INTENSE security theater North Korea, you need the JSA.

If it's your first time in Seoul, you will have many things to do. You shouldn't be at a loss if JSA cancels. As far as the "other sights" in Seoul, I can't say what you have left/what you will have already seen of course, but there's plenty to do in Seoul.