Thanks for that additional info.

Korean Air. CDG->ICN->REP.
KLM. LUX->AMS->BKK
Air France. CDG->SIN and a paid SIN->REP

I think you have a decent itinerary as it is. Siem Reap to any city in Thailand is going to require a connection in Bangkok. Europe to Siem Reap is going to require a connection somewhere.