Are you applying at an Embassy or within Thailand? If within you can transfer the 400k in time for your application (Non-O Visa no seasoning, Extension of Stay 2 months seasoning). No need to maintain any set sum until you need to be at 400k again for your next Extension.
You cannot work on a Non-OX, if if effectively a longer version of the Non-OA and is popular with Aussie citizens due to the financial requirements (Aussie bank not a Thai bank)