This may be a no brainer and that was the plan until we had a consult WITH a lawyer and was told that if we didn't do this through the court she could at anytime throw it back into the system (for whatever reason, she's a b**, or kid goes back to school). We want this official.
That sounds like a lawyer who wants to get paid a lot of legal fees to help you do something that ought to be basic. He's preying on your fears.
Because you don't go through FRO, notify your ex that child support ends as the child is grown and out of the house. Stop paying.
If the ex agrees, you mutually fill out a Motion to Change to amend the CS portion of the agreement so that CS ends, and send it to the courthouse to be stamped by a judge. That makes it official. You don't need a lawyer at this point.
If the ex objects, the onus is on her to return you to the legal system and have your payments ordered to start up again. Based on the children being grown and no longer living with her, she's got a hard hard battle to fight. You probably still don't need a lawyer at this point as the situation is pretty straightforward. I can't imagine what arguments she could use to support a position that CS should still be paid for an adult who no longer resides with her and is not attending school.
What is most likely to happen is that the ex will get your notification, and sigh, because she knows you are right, and that will end it.
Worst case scenario, she submits your old agreement to FRO so they will enforce CS, in which case you inform FRO that the child no longer lives with the mother and you are in the process of ending CS. Then take the previously described steps to end CS with FRO involvement.
Should you find yourself in court because the ex is completely intractible, you request that she pay all your legal costs, because this was completely unecessary and a waste of resources.
I haven't gone through this stuff myself, so if I have anything incorrect, others will speak up.
There aren't many lawyers who will tell you a matter is easy and you should do it yourself. Most will draw out a simple process, in the guise of being 'thorough,' so they can get as much money out of you as possible.
Should the child return to school, unless she moves back in with her mother to do so, I would suggest that you send any funds directly to her if you wish to support her financially.