The one warning I would give on RSR is that the seats are quite low to get in and out of, and if you're even remotely long-legged, the leg room requires a bit of an awkward angle which can definitely tweak lower backs. Less than a week before our last trip I fell off a friend's horse and tweaked something in my sacrum/iliosacral joint. While I found RSR itself to be no problem, it's very smooth, getting in and out was a little challenging, and there was no good angle to keep my legs. As the week progressed, I also found that bringing a sweater to sit on/provide lower back support on rides like Pirates helped immensely (although that of course will depend on what the problem is)! The drops are quite smooth on Pirates, and when the boats bump it usually isn't too rough, it's also something you can see from the queue and judge for yourself.
As for watching videos of rides on line like RSR, that almost always makes me motion sick, whereas the rides themselves rarely do! And I definitely have a new respect for chronic back pain sufferers, your resilience is super impressive!