Apply or cancel first?

I am going to cancel the altitude Qantas. I've had it for about 6 months now. It's the only card I have. About 3 months ago, I closed my Virgin Flyer.

I want to churn one more card before applying for a mortgage in the next 2-3 months.

1. should I cancel the altitude Qantas before or after applying for the next card (I have 15k limit)

2. what would be the next logical card? Ideally Qantas points but Virgin would be ok too.

3. would this affect a mortgage application? I think one recent new credit card should be OK. I approved for the Westpac altitude black in Dec 24