Flown this flight dozens of times. I generally leave central BKK at 4:30-5am. Arrive 5-5:30am via private car or taxi, check in at the NH Diamond counter (no line) and am in the TG C lounge well before 6.
The entire airport is crowded at that time. Lounge as well.
I noticed during a recent trip a couple weeks back that most restaurants are no longer open 24x7 airside, and open at 6. This really pissed me off. No more Mango Tree Thai food at 5:30am when I arrive. Lounge food is really terrible IMO, but that's just me. Yeah, no spa when not on TG.
Don't expect much. Do expect LONG walks.