If I recall, I can safely rebuild at anytime, though it’ll slow down my forum a bit?
I'm not exactly sure, but I think you can get away with skipping posts + post search index entirely, conversations + their search index should also work without a rebuild. There might be some other tasks that you could skip, but this would require a lot of digging to figure that one out and I'm positive that the other tasks aren't that much of an issue, even at your scale.
You can try to rebuild the data while the site is live, however, we do neither guarantee this to work nor do we recommend it. I'm somewhat positive that you can skip the aforementioned tasks and the remaining pieces shouldn't take too long, therefore I recommend that you take the site down (or at least enable the maintenance mode) while you run them.