As a business class passenger you should be able to use any ST lounge (which at BKK means AFKL or China Airlines).

That said, I was there this summer flying Saudia.
- AFKL lounge was very poor. Hot, minimal food, and some $10 a bottle "champagne" they should have been ashamed to put out. Went in, looked around and left.
- China Airlines is tiny, and was completely full at that time. They were willing to let me in, but warned about the crowd. I lookes inside and decideed to skip it. Food options might have been ok, but otherwise not a pleasant experience.

Miracle lounges are not great, but certainly better than either of those. At least they have plenty of seating, a usable shower and food options are ok (check both sides).