Unfortunately, you don't have much choice here. Either do KYC and send them the required documents, or don't, and thus forget about your money. Very bad choice of exchange, especially for such a large amount of money. The sooner you accept this, the less time you will waste.

Also, don't expect much from Bestchange. They are only an aggregator and forward information about the exchange rate. They do not have a special influence on the exchanger itself when it comes to the unfreezing fund, and based on some earlier cases, they did not really insist on helping users, they rather protected the exchanger partners.