its hard enough to open one account t these days. For the 800k, you need that when u apply but can go to 400k after 3 months but must be back up at 800k 3 months before you apply for a renewal.
Then you need documents to show the source of your saving. If you can show you earned those saving prior to 1st jan 2024 then if u bring them into Thailand, the capital is not taxable. Any income which is generated after 1st of jan and brought into Thailand is taxable. The most important thing to consider is how are you going to prove what you claim.