As a thai myself, this is how our country view ourselves during the war. we try to stay as neutral as possible and try to resist the japanese a bit (2 small conflicts) until the japanese bring their ships to gulf of Thailand.

https://en.wikipedia.org/wiki/Burma_Railway
In fascist tree, we can have the infamous "railroad of death" or burma railway as one of focus that increase infrustructure in the siam state.
The focus for claiming Laos and Southern Malay and Burma is a must. (some of us still bitter about it today)

the neutral tree, I suggest give them some land fort across border with indochina and coastral fort s.

will come back later when I have more ideas.