Thai has one main F Class Lounge at BKK. It is on the D concourse, (where all the shops are immediately behind security). This is the lounge with the semi-private rooms. Next to it is the main D concourse Royal Silk (Business Class) Lounge. This is huge but gets absolutely packed out between 21:00 and 24:00. QR J passengers can use this lounge.

In addition Thai has two lounges opposite each other at the start of the E Concourse. One used to have a small, frankly pathetic, F scetion but I think that may have been closed. With substantial "efficiency" savings over the last couple of years it is rare that both these lounges are open at the same time. There is a furtehr Thai business class lounge at the start of the C Concourse. I believe QR passengers in J can use any of the Thai business lounges but the most convenient would be whichever of the E Concourse lounges is open at the time as my experience is that QR flights leave from E Gates.