Just won my second game as Indonesia (it's amazing how you can faithspam Great Scientists with +186 faith per turn, in part due to four candis each churning out 6) - a game I deliberately played on Pangea to test a point. I find Indonesia much easier to play than the other BNW civs I've so far tried (Assyria and Morocco).
I find it strange to look at the UA and think "how restrictive" without considering quite what it does. As noted, it's up to 12 happiness + 21 gpt. Settling one off-island city (which is almost always going to be possible, usually even on Pangea) is almost a UA by itself - two (which is typical in my experience) is strong. The situations where you can found all three are slightly less common, but in those cases Indonesia is grossly overpowered, so it should be rare (in fact, it's at most uncommon on shuffle maps).
There is no real need to settle your own luxuries while there are plenty in the landscape unless you have a particularly attractive site - sure, scout out and stake a claim to areas early in case someone else shows an interest, but there's no need to rush even Optics, let alone Astronomy.