I don't get the craze with Japan actually. Almost every Singaporean has it as one of their top destinations. I guess that's why they are making zone 7 more expensive.
That said, I can see plenty of availability (up to 4 saver) on SQ638, and some availability on SQ12, both T+7 from today. Generally the most availability happens either 1 year out or 1 week out, which does not suit those with a fixed schedule. If you cannot plan way in advance or travel at last minute's notice, or just unable to find saver space, I do suggest changing your strategy and go for cashback cards instead. This way, you can use the cashback obtained to buy a revenue ticket, which has no such restrictions.
Another point frequently brought up is that people find red eye flights less 'worth' to redeem for, because you would be sleeping. First of all, the biggest difference between J and Y is the lie-flat bed. It makes a whole lot of difference getting that few hours of rest, so you won't feel fatigued after the flight. I would actually prefer red eye flights over regular flights. Second, my argument will always be that it is better to fly on a red eye flight than to pay extra for standard instead of saver on a regular flight.
Lastly, 6 hours is not exactly that long that you are forced to fly in J. I am perfectly fine with flying Y for that duration. Why not consider making a confirmed booking in Y and waitlist for J on the same flight. If it clears then good for you, if it doesn't then you're not exactly losing that much. At least you still have a seat on the flight and your trip will still go as planned.
