then where does my Joint Decision-making authority come into play?
In general, the primary custodial parent has the authority to make those decisions. "Joint Legal" doesn't really mean ""Joint decision making authority". It means you have the authority to make the day to day decisions while the child is in your care and you have the legal right to obtain the documents relating to education, religious and medical information of the child. That wouldn't be so if mom had sole custody. You would have to ask her for such information rather than being able to go to the providers themselves.
Generally, primary custodian will have the right to decide which school or church and make the big decisions on medical treatments.
As the child gets older and the instability of changing schools shows in his academic records then you may have a case relying on the Best Interest standard.