Economy RTW tickets book in to L inventory, so as you already know, that means no AA mileage credit for CX flights and 50% for QF flights.

You're allowed a total of 16 flight segments, so most people use all 16 to maximize the miles/value. You can have up to 6 flight segments in North America and up to 4 in each other continent (Europe, Asia and South Pacific), though as you're starting in North America, that also limits the number of USA flights (as flights in the continent of origin are more limited). Depending on the itinerary you want, you may want to consider starting in Europe, with a positioning flight to get you there (and back eventually). The RTW ticket itself may be cheaper starting in Europe rather than the US. You can stop and start as often as you like, subject to flying the last leg within 12 months of starting. Date/time/carrier changes are free but routing changes are $125 each time you change the routing so better to get it right first time when you ticket it.

For those legs that are on carriers that have premium economy, you can upgrade (for $) on a leg-by-leg basis, so that can improve the mileage earning (at a cost). You may want to look at flying on Finnair, as that will get 100% credit on AA. You'd need to fly via Helsinki, but AY flies to Hong Kong and Singapore and has code shares on to Oz. From Oz to LAX, you can take the AA code share (on QF). People often fly LHR-DXB-LHR to rack up the miles (albeit in J rather than Y).

You could perhaps try BOS-LAX-LHR-DXB-LHR-HEL-HKG-SYD-PER-SYD-LAX-DFW-BOS. That only uses 12 of the 16 permitted legs, so you can add legs in Europe and Asia and in Oz (and would need to do so in Oz as I don't think you can do 2 transcontinental flights).

It all really depends on where you must go/would be interested in going and whether you simply want to get there as quickly as possible or whether you have the time to maximize the miles etc. Your best best is to ask your questions in the oneworld forum, rather than in the AA forum.