Personally I don't think Pionus are really overbred - some species are, but Pionus are just not all that common, and are relatively easy to care for for a medium-sized parrot, they just tend to be overlooked compared to more colorful parrots. A small breeder raising a clutch of babies a year and providing excellent care and educating buyers is a good thing imo - the problem is mainly commercial scale breeders or uneducated/unknowledgable breeders, and uneducated buyers as well.
But in any case, you need to do a lot of research before you consider breeding and I do think it's a good idea to get one as a pet first. See what they are like, learn their behavioral quirks, and talk to Pionus breeders - try to meet some adult Pionus, too, not just babies. Learn what makes a good breeder (among other things, a good breeder will breed only from the best quality, healthiest adults in top condition, will give the babies and breeding bird proper vet care and a varied healthy diet including lots of different fresh foods, will socialize the babies well, let them learn to fly, make sure they are fully weaned before letting them go to new homes, and most importantly, work to educate and screen new owners and be willing to take birds back). Most good breeders will also maintain a waiting list for babies to ensure their babies will have homes ready.
As with most medium and larger parrots, pet birds usually don't make good breeders. Pets set up for breeding will typically become very territorial and aggressive around the nest. Some birds will revert back to being nice pets when not breeding. But in any case, it is a natural behavior for them. In the wild, good nest sites are rare and a pair must protect their nest fiercely - or else another pair might move in and toss out their eggs or babies. That means that even other Pionus that they flock with outside of the breeding season can become a serious threat when they are breeding, and they often see their owners as a similar threat.
Oh and given a proper diet and a nestbox, a compatible pair can breed in a large indoor cage, but an aviary is so much better for them. Depending on the climate, an outdoor aviary would probably need to have a heated shelter or an attached smaller indoor section.
I don't know of any good sources of info on breeding Pionus. Years ago, there was a Pionus research group which was a group of some breeders that funded research on wild Pionus, especially their diet, but it looks like it's been inactive for many years. You might find some useful info on their site even though it looks quite dated now.
Pionus Parrots Research Foundation
