QR835 departs BKK 20.40, thus check in opens around 17.40.
I am also booked on QR833 21.15 next trip and land from UTH around 17.20, ideal to check in around 18.00. Previously QR833 departed 20.45, I guess it will not change much for me.

But lounge food at Royal Silk lounge is minimalist and not impressed by the CP lounge neither, I can't wait until after 22h to fill my stomach with airline food, and whilst J food might sound OK I hit the Novotel buffet which starts at 18h, free minibus at Novotel desk near gate 4 (there is a walk tunnel to the hotel at the lowest level near the Airport Rail Link but in Thai temperatures I rather take the minibus) The excellent international buffet costs 1050THB net + drinks, plenty of time to catch a minibus back to the terminal and to visit one of the lounges. Plenty of time during the flight to get airline food later in the flight... Now when QR will open it's own BKK lounge I might drop the Novotel buffet.
And No Thanks for the 3rd floor expensive lousy eateries, anything there will cost you 500B, but you still have the 100B Magic food court far West side on arrivals level if you fancy street food