Everyone and their uncle code-shares into USM and it's not cheap (as you already know) with only a few airlines' metal actually landing.
One stop options would be with TG through BKK or with MI/SQ through SIN.
There seems to be an MH/FY flight from PEN though I can't figure out who's metal it is. And I don't think it would be of help anyway.
Amex can be moved to SQ and their award chart shows 12.5K Y and 20K C o/w plus YQ and taxes et al. If you're purchasing a r/t I find an 11,800 THB fare. A o/w fare is being quoted a bit above 12,000 THB. Not sure what award availability is like using MI/SQ for this route.....