In almost all cases, upgrades are only availbale when the operating airline and FF program are the same. So if you want to upgrade US trans-contenental flights, you will have be using AA's flights and program. IUf flying to Asia, it depends on the carrier selected, and you choice really are only AA (or NRT) or CX (to HKG). And I don't know what options CX's FF prgram offers for upgrades.

The only other option Trans-Pacific is QF, but that means routing to Asia via MEL/SYD/BNE. QF offer upgrades through its FF program, but they are very expensive in terms of using points or accruing Upgrade Credits.

I think you might be best with AA given the ability to earn extra AA miles using US credit cards and other partner programs.