They might ask for 3-6 months in statements, but as long as the most recent statement shows 500kTHB in the balance you will be fine. Some embassies only require the most recent statement, usually the case if you’re applying within the US.
You just have to accept the fact that some people just won’t meet the visa requirements and will have to go through an agent. Sure they shouldn’t be here if they don’t meet the requirements but that won’t stop them and people like that are agents bread and butter.
It’s a win win situation for most people, money talks here.
Reach out to Tiktok Services it’s an agency there, they will go with you to open the bank account and handle everything in person. They only charge 5k THB which is relatively cheap compared to BKK agents and such.