Congratulations on having a bun that’s so awesomely advanced in years!
I had an old-ish dog who got laser treatment for her very bad back. That, combined with daily NSAID (non-steroidal anti inflsmmatory drug, for rabbits f ex meloxicam), it gave her 2.5 more good and active years.
Exercise is good, so its not adviced to give the pet a “too easy” life. You might add ramps to the couch or bed (if she’s allowed on the couch or bed), but you will want to encourage a bit of daily exercise, such as with a treat ball (if she’s food oriented).
My old dog got gabapentin for painful flare-ups, as needed. It’s a bunny-safe drug. It can make the pet very drowsy, though. I only used it occasionally, but many dogs and buns do well on daily gabapentin.
Older buns, especially if they’re skinny, tend to like a bit of warmth. A warm water bottle or snuggle safe heat pad to snuggle up against might be appreciated. Just make sure she has enough space so she can move away from the heat source if she wants to.
There are joint supplements. Some people give a tiny amount of horse joint supplements, because horses and buns are very similar in what food they tolerate and thrive on.
If your bun is a hard keeoer, a small amount of rolled oats and a few (very few, like 3 seeds/day) sunflower- or pumpkin seeds can be added to her diet.