Testosterone boosts your baseline endorphin levels, as well as your endorphin response to daily activities. The former will wane over time, just like the effects of opioid drugs, which will eventually diminish if you do not increase the dose. But the endorphin boost that you get from exercise or from any activities that you find fun or rewarding, that seems to remain. So even after the honeymoon phase, I am far more resilient to my chronic pain. Exercise and sex are the activities that produce the most pronounced endorphin boost, but other activities do to a lesser extent as well. This helps explain why guys notice, for example, that music sounds better.
Better sleep and a lessened emotional response to pain are other enduring benefits.
Fish oil is good stuff, but it eventually caused gastritis for me (with bleeding in the stomach confirmed). Omega 3 supplements have both the benefits and potential side effects of NSAIDS.
I am glad to hear that you are doing well with TRT. It is not going to cure serious cases of chronic pain, but I have seen significant enduring benefits. Either way, let us know how it goes.