I am in Bangkok right now.
I arrived at 11:40pm yesterday and I could check in my hotel(Emporium Suite) at 1:10am.

If you flight is 8:40am, you have to leave a hotel around 6:30, means wake up around 5-5:30am.

You have no time to enjoy hotel nor suite in any hotels in the city.

What you can do is to take shower and 3 hours sleep.

Therefore, I recommend to stay near Airport.
Amari is usually convenient but expensive.
There are several hotels near airport.