When to book

Looking at using points for a return trip BNE to BKK around new years. Booking Singapore Airlines via Velocity there's nothing in business but 35000 on economy. Is it worth waiting for a possible release of business seats or should I be jumping on the economy while it's there?

Business is way better value at 75000 + $500 as with economy I'll be paying $200 +$200 for legroom seats.

Any rhyme or reason as to when these are released?

I have alerts set on seats.aero

Thanks.