If you plant the tree, you can do what you want with it, you took the time to grow it out and you are not negatively impacting natural environments. It's a bit silly to destroy something that will give you a lifetime supply by killing it for one extraction though, ya know?
Trimming a tree back doesn't kill it (if done properly) but taking it's bark generally does. When it comes to wild trees the less invasive you can be the better. I'd advise against taking any living material from wild trees if you can get away with just taking fallen material. If you do take living material, leave the bark alone and stick to leaves and twigs/young branches, and remember the wildcrafter's rule: don't ever take moar than 1/3rd of the material from any one plant and no moar than 1/3rd of any stand of plants so that they can properly repopulate.
Plantation methodology can work, and I know in some places mimosa is grown this way. However, these trees make a great long term addition to yards/gardens and especially permaculture environments, since they are nitrogen fixers and will feed other plants enabling the creation of healthy soil and promoting long-term development of robust ecosystems.
Being respectful of the trees and allowing them to live their full lifespan is beneficial on every level and in a very holistic way. Selfishly, it allows you have to a sustainable supply of the alkaloids you seek that you can continuously return to without having to constantly seek new sources or plant new seeds. In the bigger picture it's a small step at creating and maintaining healthy soils, ecosystems, and personal relationships within the wider context of your local bioregion.
Be nice to the trees and they'll be nice to you.