Ah, I genuinely forgot about purchasing maps. I've always preferred stealing them because it's free and prestige is generally better used for vassal loyalty or disinheritance these days. Of course, I'm not usually playing campaigns with a 40 year time limit, which is where the disadvantage of espionage becomes a factor.
The stability cost -4 was for converting to Hindu, which
@pratik_maitra was thinking about. I think it would be difficult to convert to Fetishist as Ternate, because you're going to need a powerbase in Asia before you can get into east Africa, and rebels won't convert your nation unless >50% of your provinces are the new religion. Intentionally letting rebels siege your nation also ruins all of your prosperity. Maybe not impossible, but I would be incredibly impressed if you could make this work within the constraints of 40 years.
On the subject of prosperity, you're right that it depends on your ruler, and it's why resetting for a 6/6/6 is the single best way you could improve this record. Not only because it would give you more development, but because you would have more time to conquer land. That being said, if you rely on a 6/6/6 for Ternate to accomplish what Mahafaly already can, Mahafaly could meanwhile conquer Ethiopia's two extra gold mines with the same luck. Comparing the starting rulers is kind of pointless, incidentally. You're obviously never going to keep your starting ruler as Mahafaly. And with anything less than a god heir that you abdicate for, there isn't time for Ternate to accomplish prosperity. I just barely managed to get prosperity in all states in time with a 4/3/3 as Mahafaly, on a very strong attempt. You also have to factor in not just time for prosperity itself but also time to reduce devastation (30 months to build fort+tick down) and core (3 years), which adds ~6 years independent of your ruler.
I already analysed prosperity and talked about this much earlier in the thread. There was a reason why I considered Mahafaly the best, but nobody has given me any credit and just assumed that I took 5 gold mines and went AFK. Even someone who copied my strategy exactly, step by step, didn't understand any of the planning that went into it and thought it was only good because of easy micromanagement. I wanted to help people understand a more powerful approach to economy and the game in general, but at this point I deeply regret sharing my strategy or trying to discuss it in this thread, where almost everything I've said has been largely dismissed or ignored.