Just a small update.
I've now made three (minor) camps. Don't know if they'll come to play at some point or not. Today I tried to expand Lonecanyons, but seems like I'll have to ditch it, since I think I encountered the beginnings of the animal population explosion bug. Sigh.
But good thing is that it looks like before today's work the site was ok (edit. thankfully I've been making lots of backups). I'm also starting to think that the biggest issue might be subterranean animal people and not other animals. Some testing with exterminate showed that they're the ones doubling even after it, but wild animals seemed to be kept in check. If this is the case, I'll in future games disable the subterranean animal ppl civs and make them just a kobold-like cave civ. Or just regular animal people without civs. (But yeah, this is mostly just a hunch and most likely I'm wrong.)
Also did a test to make a proper fortress on top of Hellsieges and in fort mode I could exterminate the insane amount of tigerfish, hippos and whatever to end the lag. Didn't try to retire it and visit in adventure mode and see will the issue re-emerge. But having a fort on top of a named camp/another site has it's own weirdness, so it's not much of a solution.
I might take some time off Waterlures, since having to scrap a whole days work sort of killed the mood, but we'll see how I feel tomorrow.
Oh, I also tried out "road building" by making unnamed adv camps next to each other. Works otherwise fine, but the fast travel map will naturally have a weird line of camps blocking what terrain lies there. So, might be something to try out with two sites that are almost next to each other rather than make a long "road" (where you couldn't fast travel anyways).