Best approach for booking Flights with tentative return dates · Travel 4 Less

Best approach for booking Flights with tentative return dates

I have a question that I believe many people would love to know the answer: when booking a trip where you do not know the exact return date what is the best way to typically approach this?

Further context on my situation:

I am travelling to Bali in June and want to try to book as early as possible.
The fact that that has stopped me so far is not knowing my exact return date with some uncertainty around when I may be needed in Australia during my planned return in late July.
I also recently just got a personal AMEX to maximise my rewards as per the Travel blueprint.
I am somewhat stuck around how to decide which airline to choose and whether to buy a Flexi ticket or simply buy each leg one at a time.
Is there a decision framework to help make a good decision around this based on if someone has a frequent flyer membership or a certain amount of reward points?