CX524/CX504/CX450 almost always have the inferior regional J product. Not worth it.
CX548 is the 'prime' flight, perhaps the most important regional flight for Cathay, and the happiest FAs (longest layover, best config to work in). But the main meal is breakfast.
CX520 seems to have reverse herringbone all the time, but not as guaranteed as 548. Lunch service but the FAs are quite stressed since it's one of the longest same-day turns they do (aka no allowance).
CX500/CX542 is primarily comprised of Japanese pax so it's a lot more quiet and orderly. But reverse herringbone is a mixed bag and definitely don't count on it too.

tldr; 548 or 520.

PS you can do mixed class for revenue or Avios award booking.

PPS somehow, CX is very popular among the Japanese. Don't count on upgrades happening. The exception may be the red eye 524 but the J product is inferior anyway.