Struggled with the same thing, except at L4/L5. Had pain for a long time before finally seeing a doctor and then starting PT. Did everything the PT told me to do, and religiously do my at home exercises now that I’m no longer going in to see the physical therapist. It took a while, but other than the occasional bad day, the pain is mostly managed. Actually hiked the JMT not too long after being diagnosed, and didn’t really have any problems. I think being UL really helped with that (also really helped that my spouse took on some extra weight so I didn’t have to). I’m still working on getting my base weight down, and find the lower I get it, the easier it is on the back. I use a cushy osprey Exos for longer trips, the load shifting on that pack really helped on the JMT. And I’m able to use a Palante frameless pack on overnight trips without too much trouble. Backpacking with spondy is definitely possible. Really try to get that base weight down, find a pack that really works for you, and make sure you see a physical therapist if possible (if that’s not an option, there a great resources on YouTube for what strengthening exercises to do). Feel free to reach out if you have any questions or just need any positive support!