ᐝᐜᐝ Yeah, history is is one hell of a mess to get right, with wars being possible with the reach - I must have gone through something like four dozen regens on SleeperVolcs for the history seed it ended up as, checking legends mode on fair few to find the optimal end year date. (Hence the facepalm of using temperate instead of tropical at that time.)
They're usually not so inconsistent with my designs as I tend to go with distance/ocean separation so that desired embark area is reachable by all the civs without them warring against each other. However the requirements of The Grim Sleeper were both unsuitable for that mode of operation and supporting wars.
Even then, I've been surprised by post-worldgen course a fair few times, with stuff like elves, humans and dwarves coexisting peacefully for centuries, then latter being attacked by both of the former before I get the first human caravan.
Just curious, but what were the numbers you had for civs? I assume you increased civs for more towers, which means as far as trade and war goes you still have only the one you meet (plus, humans will totes war with each other), but not necessarily so on visitors.
ᐝᐜᐝ Sadly the other civilizations won't be using your sent donations, they're only useful for improving relations.
You have plenty of goblins, but given how tavern visitors turn hostile when their civ's army comes to attack you I bet you can't get any from their civilization, which leaves the 30 and 100 you mentioned.
Not everyone will become a visitor, only a fraction (sometimes a large fraction - I had one case where third (600) of the elves crossed the ocean to study at dwarven library).
No idea if people capable of visiting will be generated post-worldgen, but you could perhaps confirm (but not falsify) this by downloading Archcrystal save, painting a tavern, temple and library open to all and then looking if any humans visit.
ᐝᐜᐝ If there's less migrants because of the danger, it's because of the danger at your fortress, not somewhere nearby.
ᐝᐜᐝ I've seen mentions here and there that steel is bit better for rigid pieces due it's density, but that matters little in the face of ability to ditch armor user with candy.
Yeah, you can make armor big enough to fit elephantman, though might be worth checking if there are any you can adventure as first. Though in that case the candy armor will actually have some weight (not sure how much this affects, but I know size versus gear weight doesn't scale up well).
On the plus side, elephantmen are far less vulnerable to blunt damage.
Also, I'd say consider using a shield - not sure how unwieldy they are in adventure mode, but even if you plan to avoid direct combat, you still benefit from dodging and blocking arrows and bolts. It should multiply how long you can avoid being hit, starting at 2 for unskilled and going higher from there.