To add another data point for others finding this thread later using the search:
- called SQ, they said I'm already prioritized on the waitlist (*G) and this is all they can do.
- in the end, the waitlist did not clear (like SQ325 mentioned it usually doesn't for D on SQ, so +1 for that from my just one experience with it).

And by the way: I survived the early flight and was even able to nap for 2 hours on the 8h flight with the "slides" (angled flat 2-2-2 seats on A330).
Thanks everyone!