Yes, MEL-CGK, MEL-MNL and ADL-SIN are already seen as some of the early routes QF may choose launch once the XLRs arrive. Given it is principally a business route with very low tourists except for a few Indonesians coming to MEL for shopping, I would expect MEL-CGK to be more likely a QF route rather than JQ though as the products seem to converge, in Y at least, I guess anything is possible.
The market to/from CGK is very large. While likely not a lot of tourism, it's a mix of business traffic (as you note) as well as VFR. There are a lot of Indonesians living in Australia, a mix of students, short term workers and migrants - particularly Melbourne. Perception is that it's low yielding traffic though. 2019 saw 334k total pax between SYD/MEL/PER-CGK. While there is some substantial seasonal variation, that is 915 per day, on average. While the yields are low, QF can cream off the top of that yield, just like they already do on SYD-CGK. A321XLR is great for that since you're only looking to pick up a slice of the market. They'll be competing with some one-stop options though (notably SQ and TR).