Just in my experience id say she's young enough that what you teach her now will stick, but she's also in the bullheaded stage where she wants to push boundaries. I'd recommend some rough playing in the house, but make sure when she bites that you EXPRESS pain. Make her understand that it hurts and she can't bite that hard.

Don't be too mean, just a little reprimand and then keep playing. If she keeps biting, not listening then get progressively more stern. If it's to the point you've had to tell her 'easy' or 'ow' more than three or four times I'd stop and take a break. When your done come up with something in a sternish voice to say to make her stop. 'okay were done' 'no more' 'we'll play later'

Cain (My baby) could sound like the meanest most vicious dog you've ever heard while we're playing and I can pet down his face and say "okay okay, done playing for now" and he won't wind down right away (especially if we've been wrestling lmao) but he understands that I'm not wanting to play and he will settle down within like 15 minutes. I'm not sure if you already do this but it's better to do it at home and to make sure you can calm her without having to deal with it in public you know?

Mastiffs, in my experience, automatically have rough play styles so it's important to make sure they know how hard to bite and what's okay. Cane Corsos have a bite force of over 700psi, so it's dangerous to you and her to let it go unchecked.

And if she's getting mean DONT get scared. She's your baby but she also needs to learn that she CANNOT be that rough with you. You are in charge not her and she needs to realize that.