Mostly my problem with Thailand is the same that I mentioned later when talking about Unified Korea. While the Stingray and M48/60 modernizations would fit really well, the T-84 and VT-4 would IMO be another negative step towards asymmetry, in the sense that these vehicles have completely different playstyles and come from “different sides” with different design philosophies. Although there is a case to be made for them, because they are different from the standard vehicles of their type, it would still be fundamentally opposite vehicles together. Yes, they would enhance top tier, but they would, in my opinion, lessen the asymmetry that the game does so well, and in the end it would be kind of like if the M1A2T ever gets added to the Chinese tree: a top tier vehicle that is used by the country but seems out-of-place when compared to the other vehicles it is in lineups with or will be facing consistently.
The aircraft are a different story, mostly because they’re basically all American (besides Gripen), so I don’t really feel the need to touch on them given the “American” presence in the Japanese air tree already.
I still think that South Korea is the better choice for gameplay because besides the potential T-80UK premium/squad vehicle (since those are basically exempt from this line of thinking, bc you have to pay for it) basically all the equipment is of “Western” design. Thailand has this asymmetry problem that South Korea doesn’t, which isn’t exactly a negative thing because it would be something new, but it isn’t exactly a positive thing either, and it would be doing a disservice to Thailand if some of their main modern tanks were stuck as premium for that reason.