I'd say just buy for the few dates / flights you want and cancel ones you don't want closer in/refund to Wallet as it gets closer. This also has a benefit of locking in fares vs them potentially creeping up as additional fare classes sell out, or prices climb. After enough cancelled flights, you should have enough of a little cushion in your wallet so less will be charged to your card, but if buying a few flights is impacting your credit score, try using a card with a higher limit, applying for additional cards, etc etc.

Supposedly some have reported AS contacting them if it appears there are impossible flights purchased (e.g. departing an hour later, while on another purchased itinerary), but I'm not sure how actively they're checking that. All that churning between flights and all those refunds to process are probably just as annoying to AS as they are to you.