IMHO, by far the easiest thing to do is just leave the 800K THB in the bank. You then don't need to worry about any of the pitfalls spoken about in the earlier comments and the pitfalls are indeed real.
I saw this one the other day but I have no idea how reputable they are. That said, I hear people on here rave about how good TikTok Services is but I've never used them & I don't know their prices.
I didn't read all the comments so maybe this has already been addressed but technically you need to have the equivalent of 20,000 Thai Baht in cash & proof of where you'll be staying (e.g. Proof of hotel booking.) BTW,the 20K THB can be in ANY currency (e.g. GBP, USD, EURO, etc.)