I would definitely take SQ997. Note that SQ995 which leaves a little earlier is via BKK and regional first class not the skysuites (which SQ997 has).

SIN-CPT is 2-class so no first. I think that's the only longhaul flight on your itinerary without first class?

According to GCM you have 29,628 miles which is just over the limit for FRWSTAR1. Either cut 630 miles out or you can add more than 4000 miles.