You need to go to yoir (thai) bank, and make a minuscule transaction a moment before that. They will give you a paper stating when the money came from outside Thailand.
Bangkok bank knows that. I’ve been to 3-4 different branches and they immediately know about it.
From all the comments and what you answered, I see only two ways:
1. You find an online job before moving. I was lucky enough to have my home office here before I retired. But you need to already know something that supports online, like me being a senior software developer. Just starting with YouTube will not do it, bc that take 5-10 years time.
2. Get married (I suppose you’re not 50+) to a Thai. You’ll need to show
*****
0 baht in a Thai bank account or
*****
monthly income or a mix of both.
There’s enough ladies looking for a supporter, so this part is easy; but beware, you find the wrong one and get divorced quickly and your visa is gone too.
The 400k should be easy, but you didn’t say how much you have saved.