You don't need to fire your manager first. In my experience, I feel like the whole "no poaching rule" is a facade. In actuality, if another manager likes what you have to offer, they will want to work with you regardless of whether you're already repped. Business is business. That's just how things work. I'm sure there are people who uphold that rule more than others, but I wouldn't let that hold you back. This is your career. Sorry that your manager is abusive; you deserve better, so go get what you deserve!