This definitely doesn't seem like a bad trip if you've not been to China and Thailand before. I think the longish breaks on the way actually make the trip less exhausting rather than more. F on the CA 773 is supposed to be very good, though if you see NH to Japan combined with TG to BKK open up later I think that would be more attractive to me and I'd pay the change fee (if applicable). (Note, NH does not have F to BKK, so TG is your target on that segment)

For Bangkok, I wouldn't stay around the airport since you'll be there all afternoon. There's nothing at the airport to see and higher end places in the city can be had very cheaply. Several of the upper end hotels have nightclubs in basement if you want to dance the night away and there's a lot that you can see in the morning/early afternoon in Bangkok, but learn and avoid the tuk-tuk shopping scam to save yourself a lot of time and possibly money.