I want to say up front that this is based purely on my personal experience.
A carrier similar to Korea's SK is Indonesia's Telkomsel. It's the biggest one. There are also others like Indosat, XL, and AXIS. Telkomsel in particular has recently started taking a conservative stance on prepaid plans. They want you to register and use postpaid if possible... and honestly, they use a pretty sneaky (?) method for it. A data package you could buy for 100,000 Rp suddenly becomes unavailable to purchase. I really thought it was gone... but it turns out only more expensive packages become selectable. Of course, the cheap package you used to use hasn't actually disappeared, but if you keep using it, that product stops appearing.
If you're planning to use it long-term, postpaid is indeed cheaper. But you'll need to go in person to a store like GraPARI and register with your ID. (This can turn into the ultimate test of patience while waiting.)
If you're not planning to use it long-term and just want to top up as needed, I'd recommend minor carriers like XL or AXIS. At least in major cities like Jakarta, Bali, and Lombok, you'll barely notice a quality difference from Telkomsel.
The reason I recommend prepaid plans is that you can buy a SIM and just pop it in and use it right away, without a separate registration process. One tip: instead of big roads, try small USIM shops located in alley-like areas. For about 50,000 Rp, you can buy a prepaid USIM with roughly 10GB included right on the spot, with no extra paperwork needed, and start using it immediately.
Most Koreans tend to try to handle it at big malls rather than local alley shops... but if you just look a little differently, there are lots of convenient, affordable options. If you have a housekeeper or driver at home, try asking them to buy it for you. It's easy enough to buy on a trip out for a meal.
One more note — Pulsa... that is, the cash-like top-up balance also has an expiration date. It can vary by carrier, but it's usually a maximum of 3 months. It doesn't automatically disappear after 3 months. It disappears if there's no activity (usage) at all. The carrier is reclaiming the number to resell it. Set a fixed date every month, like the 1st or the last day, and even if you have no usage, buy at least the cheapest data package (around 100,000 Rp) so your Pulsa balance and number don't get deactivated.
