Did the BKK-BRU offer FC or not? If so, do you know whether it was overbooked?

You may find that the difference in price between a FC through fare and a ticket with a broken fare, FC to BKK and then business class to BRU, is small or even negative. Also, in such cases, airlines tend to argue that the downgrade compensation should ba based on the most expensive unrestricted and fully refundable walk up fare, as if you had purchased a FULL J/C ticket at the airport just before departure, rather than as if you had bought the cheapest business class ticket what was available when you booked your flights.