CBA platinum card is not the best Qantas point earn card out there. The problems are:
1. You need to spent extra $49(?) p.a. to opt in to earn Qantas points. So automatically it increases your annual fee.
2. Poor earn rate. With Amex you earn 1.25 QF points, and MC only 0.5 QF points per dollar spend. There are a lot more direct earn CC out there with better earn rates.
3. Point cap rule. If you opt in Qantas, the cap is not 250k, but 200k. This is CBA award points, not QF points, so in reality you only can earn 100k QF points, which won't give you very much in terms of flight redemption.
4. Earn rate gets worse as you earn >120k award points (or 60k QF points). After that, you earn 0.5QF points per dollar with Amex and zippo with MC.
So, at your yearly spending of $400-500k, there will be at least $200-300k wasted earning nothing.
The he first thing I would do if I were you, is to request CBA for a free upgrade to Diamond card. It has a higher earning rate (1.5 QF points for Amex, 0.625 for MC per dollar) and a higher cap at 300k per year. With your yearly spending, I'm sure you would be a VIP to CBA, and I would be very surprised if they don't give you a free upgrade. If they won't, then walk away and get another credit card with someone else that has much better earn rate and similar annual fee.
If Qantas is your goal, then a few I can recommend:
- Westpac Black Altitude with Qantas opt in (formerly known as Earth card). Also comes with Amex and MC, but better earn rate than CBA platinum (1.5 QF points for Amex, and MC for overseas, and 0.625 for MC per dollar for Australian transaction), no cap and comes with 2 Qantas lounge access per year.
- HSBC platinum visa. 1 QF point/$ spend for domestic, and 2 QF point/$ for overseas. This is as good as you'll get for non Amex.
There are a lot more options out there. Have a good browse through this website, the other one I would recommend is AUSBT.
Hope me that helps.