When you say Thais, you refer to Central Thais or to the bigger Thai group?
All who identify as Thai. This includes Central Thais as well as Northern Thais, Southern Thais and Isan Thais.
I'm not sure, but I think that in the modern times the Central Thais are taking over most of the Thai groups, and it wasn't that way in the past.
Central Thais certainly have more influence due to political and economic circumstances. Luckily culture is not a zero-sum game. Central Thai culture doesn't "take over" as much as synthesises; though the degree varies. In the North and Isan local aspects (influenced by Lanna and Lanxang respectively) remain strong within the broader Thai culture/identity.
But it is not the same thing as Thai language. Thai language is the one who is spoken among the central thais (those of sukhuthai and ayutthuta).
I've never heard a Thai (north or south or central or Isan) refer to their dialects as anything other than Thai, albeit with a regional qualifier (so the Central dialect is Phasa Glang "Central Language")
Well, Lanna was a short lived civilization.
Maybe one of the shortest ones.
Five centuries is short-lived?
If you were trying to select the greatest civilizations in histroy it would matter, but here we just list every civilization.
But you're not. You're listing every significant empires, states, cultures, ethnic groups.
First you need to define what constitutes a civilisation.