I’ve read the Secret Service uses the full auto version.
IMO the caliber is too light for home defense. You have to put a lot of rounds into a target to stop them.
A shotgun or a revolver would be better for someone with limited experience and strength.
I would echo what other people said about more common ammo. Once you drop below 5.56 you are looking at short range carbines. Maybe something in a pistol caliber would be better. A pistol caliber that matches other guns you already own.
Another option might be stripping down a short AR to make it lighter and training her to get used to the recoil. You can't get much lighter recoil than an AR, but a pimped out AR with a metal forearm can get pretty heavy to lug around. If she was shooting an AR it might not really be the recoil, but the racket from the recoil spring right next to her head that she doesn't like.