I hope the new pain therapy for your back works. Fingers crossed.
I've had back problems for over 25 years, so I'm very happy that Evenity hasn't aggravated that. I had one injection of Prolia 8 months prior to starting Evenity, and that did caused some back pain.
As far as side-effects I've experienced on Evenity--
There's been injection site swelling and sensitivity following each one. After the 4th set, there was some burning too. In each case, it went away withing 24 hours.
Right after the first treatment, I got a cold sore (someone else on here had that too), and I was ravenously hungry. I attribute the hunger to my body needing more material and nutrition to grow bone.
Following the 3rd treatment, I felt extremely tired for several days.
Just after the 5th set (two weeks ago), I got arthralgia in my ankles and in one wrist. It's a tingling feeling--not painful--but it caused one day of sharp pain in one ankle. I couldn't walk on it, but it just suddenly went away the next day. That's the worst thing that's happened.
Over the last couple of days, I've been completely wiped out again. Part of that is down to my smoke detector going off twice in the middle of the night. Haha! Thankfully, it was just a fault in the system, and not an actual fire. I felt like crap the next day, and I've been abnormally tired ever since then. I'm pretty sure Evenity is playing a role in that.
Overall, I'd say that my side effects have been minimal.
I was super anxious about starting the treatment too, but it proved to be no big deal (with the exception of the ankle pain). I'm thrilled with the results so far. My spine went form t-score -3.7 to -3.2 after only 3 sets of injections. I also got my BTMs tested around that time, and they were great. P1NP = 102. CTx = 355.
It's more likely than not that you'll be fine on Evenity. Most people, if they have side-effects at all, have mild ones.
Even if Evenity doesn't work for you, it's easy to stop the treatment and go another route.

I look forward to hearing about how it goes.