I have a few more data points to share from my recent Garuda flights.

From CGK to DPS I could upgrade to C for a very reasonable 19 euros. Business class meant a flat-bed seat with a 3-course meal, excellent service. I uograded from a B fare. You need to ask at the customer service desk in CGK.

Lounge access is available to all ELP regardless of cabin class when traveling domestic.

From DPS to SIN Garuda was asking from 420 euros for the upgrade. I declined.