I've created a forum to replace an old phpbb forum. Many of the old users want me to import old posts/thread to the new one. So, I want suggestions how I can do that. Perhaps manually if it's a good solution.
What's your overall goal? I have a feeling that you are giving AWS way more money than you should already. If the goal is to go big without wasting money, this isn't the approach. Unless you are doing something in the billion hits a month range, I don't think that this approach can be cost effective at all.
So if you want to give AWS money just for fun, then you might as well give more. If you want to be cost effective, then setting yourself up for scaling will require more scale than will ever be practically needed.