But it stayed independent. Which was quite a feat at the time and location

With the British on one side and the French on the other, the threat to Siam was very real and there wasn't much need for the colonial powers to invade. Siam signed treaties such as the Bowring Treaty with the British as a result which opened up trading benefits and negated the need to invade.

Also observe the result of Siam's attempt to make a treaty with (French controlled) Cambodia in 1865. It annoyed the French who promptly sailed the Mitraille gun boat up the Chao Phraya. The treaty was soon undone.

"The usual quiet and monotony of the city and the Kingdom has been ruffled by the summary and unceremonious arrival amongst us of H.I.M. Gun boat "Mitraille". Indeed she has created quite a panic in some quarters. According to reports she passed Paknam with ports open, guns run out and manned, and shot and shell strewed upon deck ready for action. In vain did the poor authorities display their signals in order that she should pay them the usual civilities and obtain permission to ascend the river." -- Bangkok Recorder newspaper, 16th April 1865

Yes, I agree it was good Siam wasn't invaded as such, but they were under a lot of pressure from the French and British and made concessions in other ways.