What I've done a number of times is to get a day room at Novotel Suvarnabhumi (they have a shuttle but you can also walk over from the terminal - around 10min through the basement level).
Given the late departure, probably would need to pay for 1 night. Prices are usually pretty decent in my books.
You can also just pay to use the gym (incl. pool). Can't remember the price, but I thought it was a good deal.
As for USM-BKK, I've only done this once and the flight was on time. I've never had issues going through BKK on separate tickets. Everything has always worked well (whether it's been 2-3h between flights or more).