30% of people in their 70's have stage-3 CKD (raising to 35% in their 80's). It needs monitoring to check there's no resolvable cause and it's not getting worse, but it generally has no symptoms. It corresponds to 50% kidney function, or having just one kidney, which is not normally an issue.
No drug has ever been invented which doesn't have side effects in some people. All the PCa treatments available have significant side effects. You don't know at the outset which side effects you will get from which treatments which is part of the treatment choice gamble, but you can be reasonably sure what happens with no treatment. Treatment is generally a case of swapping some severe symptoms or outcomes for less severe symptoms or outcomes, and sadly not a case of restoring you to the state of never having had the problem in the first case.
You survived PCa for a long time so far with your treatment choice - many people here would wish they were that fortunate. It could have been that if you'd had a prostatectomy, some micro mets would have been left behind and you might not be around today, whereas the brachy zapped them. I would not assume you made the wrong choice, but you do need to be realistic about side effects of all treatments.