So now I conclude that directly upvoting your own post is useless if you want to earn some of the curation fee from your own post.
Yes it will go directly to your post rewards instead. If you vote at 15 minutes, then 50% of your potential curation rewards will go to your author rewards, etc. If you are the only voter and you vote at 30 minutes, you'd receive 75% of your vote as author rewards and 25% as curation rewards in SP.
Considering SBD is valued more than SP - the blockchain sees SBD as $1, so it gives half of the author rewards as SBD and half as SP, but the SBD are worth about 1.7x more than SP so its not 50-50 because someone is buying SBD above the price of $1, it means you'd make more rewards by voting on your post as early as possible and then trading your SBD for Steem or other currency.
What I meant is that if you self vote your own post directly at 0 minutes and 0 seconds that you won't get any additional curation fee. That's because you basically don't get any curation reward from the upvoters behind you. Plus of course what you explained that the reward is a full author reward.
I now doubt if I'm right about the first part of my theory. If you look at the pay out below, I know this author always upvotes his own post directly with approximately $8 upvote value. That leaves $10 as remaining author fee. So I would expect to see a maximum of $2.50 for the curation reward. But the curation reward is only $0.64. This very low curation reward should then be caused by many other upvotes in the first minutes. That's not impossible, but it does make me wonder if I fully got it or not?
Yes, if most votes are done at the beginning of the post there will barely be any curation rewards for it unless someone votes later and receives those rewards themselves.