Yes, BR Diamonds would get op-up far more frequently than OAL *G. I don't know however how often they upgrade other than for op-up, that would be a question for some BR regulars, not sure how many Infinity Mileagelands users though are on this forum.

Have a look around, you may not be locked into a BR credit card. In Japan we have cards for quite a few foreign airlines as well as Amex (standard and SPG) and Diners Club which both have multiple transfer partners. I would assume that Taiwan is similar, especially on the Amex scene. Amex is also one of the easiest cards to get for a new resident in most cases since they have the international transfer program, however you usually have to have had an Amex in your previous country for a certain amount of time first before you're eligible for that.