My current fortress has achieved Expert in most of the major cavern creatures, including Jabberers, at 12 years in. Only have two cave dragons though, both female, and sadly a Forgotten Beast rampage appears to have killed off most of the local population of Giant Cave Spiders. Managing to catch a breeding pair of Jabberers contributed greatly. The greatest training knowledge increases appear to come from animals being trained from the wild state, so keeping non-Tame adults caged but flagged for training produces the fastest results, relatively speaking. Caged creatures can't be trained until their training decays back to wild, so there's no need for micromanagement or FPS loss either. Just be sure to let trained ones out to breed once in a while if you want to keep the population up, and be careful to cage new births before they revert to wild.
Crundles are extremely prolific, as I'm sure you've noticed. I have a cage with something like 400 wild crundles stuffed into it, as I've already got enough food for the next millenium or so. Every spring the handful that die of old age get their corpses dumped out and recovered for butchering. Of note, fully wild (untrained) creatures that die of old age can be butchered, unlike tame or trained (stray X) creatures. One of my vaguely-formed future plans for a fully domestic leather industry involves breeding Giant Olms/Toads, letting them revert to wild, then using a drop shaft to splatter them for increased yield. Admittedly, I'll probably abandon this fort due to FPS death and boredom soon.
No idea about diffusion of local training knowledge back to the civilization, as I'm loathe to retire my fort to check. Among other things, I'm more than a little worried about what will happen to my chained dragons, assorted trapped Forgotten Beasts, and the aforementioned cage of crundle holding.
Oh, also, wild members of domesticated species (like the aforementioned turkeys) can be fully tamed as adults. One training session will bring them all the way to Tame.