Suddenly a barker at 6 months

My six-month old is suddenly a barker. She went from occasionally alerting at other dogs through the windows, or on a leash, to barking at the occasional humans AND dogs, to now barking at humans, dogs, the wind, leaves, her imagination. And it’s loud, insistent barking that is very difficult to make her stop. I can usually get her to “walk on” (my “leave it” phrase for when we’re on a walk), although she’s even getting more persistent during walks. It’s not vicious or particularly aggressive, more like alerting, but she’s got a big voice, and I’d really like to train her to stop when I tell her to. Any tips?

If it matters, she’s a mixed breed: pointer, cattle dog, pitbull, Great Pyr, according to her DNA. She normally has a very laid-back, sweet nature (except during puppy witching hour, during which she becomes a black-and-white spotted land shark.) She’s also been fairly easy to train, wanting to please, responds well to praise and/or food rewards.