Read the links in my signature then read them again.
It's understandable that you would want 50/50 with your children post divorce. There are basically two ways to get that.
1) Your wife agrees to it.
2) You have a documented history of providing 50% or more of the daily hands on day to day care of the child.
WIth 1, your chances of getting 50/50 are good. With 2, your chanced depend upon the quality of your documentation and your judge.
SO, for starters, are you currently providing at least half the day to day hands on care for your child? If not, you should start doing that and build a documentable history of doing that before you even consider filing for divorce.
2) to let my STBX know I am serious.
If you are going to divorce her, what do you care what she thinks? [Rhet}
You will also need to work on Radio Silence (Not tipping your hand) and disconnecting from her emotionally (not caring what she thinks).