Rotating them every 2-3 months would be more appropriate. Even though they would be "old" raids for me (and presumably you) there are still going to be people climbing up from humble origins and it will be more tempting to upgrade for a particular raid if you know you've got a month to upgrade a whole team and then another month (or a bit more) to enjoy the benefits. Otherwise the raids won't be enough of a motivation to upgrade toons.
Sure, maybe you get the same use out of the newly elevated toon per year, but that's still not going to motivate the newer/mid players. It's not that you're going to get use out of it (or not) but that you'll get MORE use out of it than some other toon. If all the raids are rotating quickly through then there is nothing about the raids to tell you which of the many toons used in the total set of raids is better than the others. Where if you know it's one raid for a couple months, then the raid is much more likely to be a tie breaker in which toon should be upgraded next.
Weekly rotation will breed roster complacency. Monthly rotation less so, but it's still a tight time frame to upgrade a whole squad and then get some use out of them. Bi-monthly is probably about right for the scenario you're portraying.