My thoughts:
I wouldn't bet my job that the age discrimination laws will happen in HK. However on bases it's more likely, but command slots may be hard to get and you may find yourself at 55 still in HK, or as a FO on a base.
I'm not sure how some of the calculations of others are done, but by mine the BPP is only worth less than a year of a CN1 salary.
With all new joiners on COS08 and RA65, once you reach 55 you'll have many junior officers to you that may not be interested in fighting for you to keep working and delay them further. Food for thoughts.