That's about it. There is also a Garduda lounge and another lounge that SQ uses (Esplanada). Most of them double as Indonesian bank lounges and third party programs (Priority Pass, Veloce etc).

Been a while since I've been through CGK but from memory the Premier lounge was slightly better. Keep your expectations very low however (and you may need to ask for alcohol). Note that even the home Garuda lounge is pretty basic.

I would say plan to get to the airport last minute, but in Jakarta that is dangerous thanks to the vagaries of traffic.