Listen...this is a long trip...no need to max out the mileage IMO. If it was me, I would fly either TG LAX-BKK-DPS-BKK-LAX with CO or UA getting you to LAX, or I would fly SQ XXX-SIN-DPS-SIN-XXX from your nearest SQ gateway.

I flew Y cabin from LAX to BKK in January, and it was very pleasant. I have had J flights that were less pleasant with US carriers. Alternatively, you can book Premium economy from LAX-BKK which is another step up.