It sounds like you are already booked, so I'm a little confused on your question.

You could have flown from Europe to Siem Reap for 200k miles in Premium Economy, then you could fly from Siem Reap to Phuket, then you could fly to Bangkok, then you could fly back to Europe.

There are a TON of flight options to get from Europe to SE Asia and you don't mention what airline alliances you want to fly with.

Siem Reap is the bottleneck in your flight planning.