We just got Ari (13.5) a Ruff Wear harness. The things are made (built?) for search-and-rescue dogs. They have a handle on the back that lets us support her back legs when necessary (not often, but often enough). You can also hook a leash onto a loop at the back of the harness and support a dog while it walks that way; there’s a German shepherd dog who has significant back-leg weakness who takes daily walks in our neighborhood park that way. Poudre Feed has them, and you can take the dog in the store to try them on.

Nice trick to know about for dogs who refuse to be left behind or slow down.

Probably not compatible with the backpack, though.