Lots of useful info in your post, but did you overlook my point that it was paid flight in F from HKG to Taipei that appear to be reducing from daily until end-October, then down to 3-4 per week until end March, and after that none? I can see paid economy and business class availability well beyond April, so was wondering why I couldn't see any First class seats beyond the end of March.

I know from experience that Cathay have a habit of making last-minute equipment changes (I have had flights in F downgraded to J before). My strategy would be to book First on a flight that also had Premium economy on the basis that such flights were less likely to get changed at short notice having 2 cabins needing to be rebooked.