The 5 year thai license is legally valid in Cambodia cause of ASEAN, but if ur stopped ur going to get fine no matter what license u show them < its Cambodia :-)
ON my many trips since 2004, i'dsay they fine me 75% of the time, showing my thai iicense
Thats only like 6 times in 19 years
Never seen any reason to pay 500 baht for a 1 year IDP when my Thai license is usually valid for 5 years 11 months and 26+ days for only 515 baht ( i always renew at least 1 day after it expires)