I know of several serious injuries, including broken backs, due to using a leash and harness. I suggest you do serious research on this subject.
Following is a post from the NYC chapter of HRS.
“Harness walking is an extremely dangerous practice that has ended in tragedy for many, due to the dangers posed by the harness itself and also the significant hazards associated with being outdoors, even for a short time.
Outside, rabbits are exposed to parasites and diseases carried by other animals, they are prone to insect/tick bites and fly strike, and they can be poisoned by pesticides, herbicides, and fertilizers. Rabbits can suffer fatal heart attacks out
of fear from merely seeing a predator (dogs, cats, hawks, etc) or hearing a sudden unfamiliar noise (cars, lawnmowers, other animals) from which they cannot escape. It is also vitally important to keep rabbits in temperature controlled areas; hot weather is
particularly dangerous for rabbits, and heat exhaustion is a serious emergency. Should a rabbit get out of a harness or off a leash, they are often not easily approached and can run and hide in fear. It is often extremely difficult to safely catch a rabbit
that is loose outside, and domestic rabbits have no outdoor survival skills.
A rabbit that becomes frightened and panics while wearing a harness can easily become tangled, at which point they could cut off their own airway or break their back or other bones. If you have a rabbit, you are aware that when they hear, smell
or see something that concerns them, they will quickly “dart” or quickly run for even just a few feet. Because of the incredible fragility of their skeleton, this movement while attached to a harness/leash will result in a broken back or neck. Even the stress
that rabbits experience from being put into a harness can easily lead to serious GI upset. Remember that for a prey animal like a rabbit, anything designed to go around their belly, under their chin, or around their neck can mimic the sensation of being grabbed
by a predatory animal, and because rabbits’ bones are so fragile, even the process of putting on a harness can easily result in unintentional injury.
Fatal accidents and illness due to harness walking are tragically common. Please provide safe, indoor enrichment alternatives and do not risk your bunny’s health and safety.