Trans rights are a mixed bag everywhere, no place is ideal in that regard. That may change during the next few decades, but rn there's some massive downsides about everywhere, even in places that are generally very inclusive. Due to the region historically recognizing more than two genders, Thai society is very accepting of trans people from all i've heard. There's no gatekeeping around trans healthcare, they have world-class surgeons for trans-related stuff and while you'll pay out of pocket for anything, the prices are more affordable than elsewhere. You can also just go and change your name for less than 2$.

The big downsides are no gay marriage and no legal gender changes. Like, at all. In the eyes of the state, you're permanently stuck with your assigned gender at birth, and if you ever get in trouble with the law, that means as a trans woman you will end up in a men's prison. Which will be a gang-ruled hellhole of a prison as well. I don't know how they deal with people who already have their legal gender changed, but i wouldn't necessarily count on that working out if you do not pass 1000%, and as an immigrant you can get into trouble with the law in a lot of additional ways that people normally don't even think about.