I know it adds to the hassle, but how about MH via KUL? This one is available in booking class Y (3 seats left), albeit pretty pricey as full fare.
19:30 Changi (SIN)
23:10 Suvarnabhumi International Airport (BKK)
Total duration 04h40m, 1 stop
Malaysia Airlines (MH616)
Malaysia Airlines (MH796)