Thanks @hike for a detailed reply.
I guess valuechampion's site is the only one that said 100,000HKD spending max, which now I realize is definitely incorrect. PrimeCredit's EarnMore card has a basic unlimited 1% cashback and a bonus 1% on the first 200,000HKD annual spending.
I'm not actually spending money as I'm simply moving my money around via physical octopus cards with AAVS, and at least 6,000HKD can be moved for free per month to my bank account (again just moving money around to earn 120HKD cashback per month). The only part I'm 'spending' on is to pay my IRD tax bill that I haven't figured out any other way to earn 2% on paying.
I'm fairly sure I've claimed all deductions I legally can to reduce my HK taxes, but luckily my salary is nice for me as a native English teacher on the Education Bureau's NET scheme (remuneration is public information). I don't have a business, and as a US citizen any self-employment income is legally subject to some ridiculous US taxes. I've deducted the standard 18k for my MPF contributions, 60k for my TVC contributions, 120k child deduction, two 50k parent deductions (my wife's parents), 50k mortgage interest deduction (I should have convinced my wife to just put our property and mortgage solely in my name, but oh well), and a bit for a small charity donation to WWF that makes my wife happy.
If you have other ideas to further reduce my HK taxes I'm open to suggestions. Previously I related back my gratuity as it lowered my total HK taxes, and I claimed self-education expenses when I had them. I also always elect for a joint assessment (I love how the IRD will only do a joint assessment if it lowers your overall tax liability), but it has only reduce my total tax bill (my wife's plus mine) once or twice.