In answer to your upgrade question, I think it's unlikely you could get a round trip upgrade for two for 110k Amex points. Most European carriers have a lousy, or even no, upgrade program.
In fact, 2000 Euros for a round trip business class ticket from BRU to HKG sounds like a pretty good price, it's probably at that low level because you are buying well in advance and travelling during the holiday season.
In most countries, you can use your Amex points as a credit against the total if you book the ticket through Amex travel, that might make it work for you depending on the "exchange rate."
Of the three cities you name, Bangkok will clearly be significantly cheaper than the other two for land costs.