Your lower back pain could be kidney pain which can definitely be related to bromide detox. If this is the case, salt loading should help pretty quickly. But you'd have to repeat if regularly because the bromide keeps leaching out of your tissues into your kidneys.
If I get lower back pain it is either that (kidney pain) or low vitamin D.
I have a copper and zinc imbalance but never linked the kidney pain to copper before. I read that copper can accumulate in the kidneys, but it could also cause trouble while detoxing copper via the kidneys (pee), so that might maybe cause kidney pain.