Well, out of curiosity (mostly to see how little conquering I could do without getting bored), I gave it a shot as a custom nation (with no allies the entire time to make up for the custom ideas). And while I didn't go the ENTIRE game without conquering, I DID stay an OPM via making client states the second I made peace with my enemies. And this is my result.
If Port Royal sank because of the buildings being too heavy, I don't think the island should exist at all in this case.
Ideas. All in all, the naval aspect was a bit too OP, but at the same time, I didn't really need them. Maritime + Naval alone would have left me as uncontested.
Subjects I gained. Any money they give me is almost instantly returned to them to pay off their debts and keep them loyal.
And the economy. With a navy like mine, extorting trade power and money from the surrounding nations was child's play.
All in all, I'd give my OPM only game a 3/10. I went navy only until the 1690s, then built one to siege down capitals. So the naval only aspect was interesting for the first war or two, but after that it was just meh. If I was on the mainland, I might have had a different experience, but I really didn't want to bother with AI allies. This was also an ally-less run, as previously mentioned. But the big killer was just the lack of anything to do. Watching your money go up loses its luster after the first century.