ANA counters open 3 hours prior to departure at Bangkok:
http://www.ana.co.jp/wws/us/e/asw_co...nter/open.html
ANA does use Thai Airways lounge. Just FYI, you will not have access to Thai Airways spa. That is reserved only for passengers departing on TG.
You should not have much of a wait for security/immigration, especially if your flight is coming up soon (low season).
When you should arrive at the airport is a bit dependent on personal preferences. For the early ANA flight, I tend to arrive around 5 AM...but I am pretty risk adverse.