To follow up with this thread, I hit a new 10,000HKD monthly limit I wasn't previously aware of. I now see you can read about it at under #4, and then by hovering over the question mark after "Monthly fund transfer limit from Registered Octopus" at https://www.octopus.com.hk/en/consum...out-oepay.htmlYou can only transfer in up to 10,000HKD in total per month from Octopus cards into your O!ePay account.
If your HK tax bill isn't too high and your AAVS credit card issuer normally doesn't let you earn points/miles/cashback from paying HK taxes with it, you could transfer up to 500HKD per day using AAVS or 1,000HKD per day if you have two Octopus cards with AAVS (maybe your spouse's card if they'll let you or maybe you have two+ yourself). You can check out more details about what you can do with AAVS with different banks/credit card issuers at https://www.octopus.com.hk/en/consum...g-methods.html I personally haven't tried registering multiple octopus cards in my name with the same financial institution, but that might be worth trying for you.
Anyway, one game/trick is to essentially add as much money as you can using AAVS with an NFC-enabled Android phone into your O!ePay Pro account (do all the things necessary to upgrade to O!ePay Pro). Then you can use your O!ePay balance to pay your HK Tax bill (just scan the QR code on your HK tax bill within the Octopus app), or you could at least pay some of it (I haven't tried a partial payment yet). Going through all these hoops means you earn the same amount you do on AAVS (2% with a PrimeCredit EarnMore UnionPay card) as that amount you can pay the IRD with your O!ePay balance (that you added to using AAVS).
I also believe paying your IRD bill doesn't hit the same 10,000HKD a month limit as the one I did when I just added to my O!ePay account and then transferred my O!ePay balance out to my mortgage linked savings plan, which I did to earn a tiny bit more on interest (H+1.24%) until my AAVS credit card payment is due. This is probably not worth it to most, but for me it's an amusing game.