I would fight the 50/50, as well. You have been the children's primary caregiver (I'm assuming), so it's often in the children's best interests to continue on that way. Does your ex even want that? Tell your lawyer that you prefer to customize your child custody arrangement, thank you very much, and not just go along with current state recommendations. If the lawyer can't think outside the box and at least try for you, then get a new lawyer. If worse comes to worse and ex gets 50/50, you can draw up a plan that eases them gradually into it, instead of all at once. I also was going to recommend the "first right of refusal" clause, that if your ex needs childcare, you have first dibs on the children.
It IS insane that these guys can just up and decide to ditch the family scene - and are promptly rewarded for it. A "good father" doesn't ditch the mom.