The thing is that it's hard enough to keep a bird healthy and happy so you should start by learning to do that - and that takes a few years in itself because anybody can keep an apparently healthy bird for 4 or 5 years, even if they don't do all the right things... Then, if you still want to breed them, you should not only read everything available about breeding but also about their physiology, diseases, behaviors in the wild, etc. because you will need this knowledge to do a good job. After that, you should find yourself a mentor because there are lots of things that you simply cannot learn from the written word, you need to see somebody doing them and then do them yourself with supervision from somebody who knows (like the proper way of catching, toweling, medicating, handfeeding, etc). And last but not least, you need to take into consideration that there are thousands and thousands of tiels in rescue (even more than budgies, if I go by the rescues I know) and producing more when the market is obviously already super-saturated does not seem like a very good idea because it's not that easy to find good homes for birds. Everybody wants a free or cheap one and everybody promises to take good care of them but few people actually pass the test of time (and tiels last a good long time if you treat them right). Especially tiels and budgies who, as you mentioned, they are considered low maintenance -something that, when it comes to birds, doesn't even exist.