Your calculations are right. Any WT+ fare is eligible to be upgraded this way, but only if there is award availability. This will be your main restriction because award seats to SIN are very tight. Try a straightforward "Book with avios" search to see available dates (you will book differently, but from the same inventory).
Alternatives are Kuala Lumpar (also 35k) or Bangkok / Hong Kong (30k) or there's also Seoul/Tokyo/Shanghai/Beijing further afield.
You can do it in one go online if there is availability, or you can process the upgrade later if availability opens up but this is higher-risk.