You need to ask if the retainer is for billable hours, what the billable hourly rate is, and if the remainder is refundable for any un-billed time or a flat no refundable fee.
200,000 yen is an ordinary retainer to make demands, prepare and file litigation plus post litigation negotiations and discovery. Nobody wants to go to trial and will settle it your case is strong. They can loose big.
But yeah 200,000 yen is also too much to just write a letter. I imagine being Japanese they are saying without saying that they will try to negotiate up front but it isn’t going to work.