Deathly Hallow Split
I agree that ending after Ron's return would be a good place to stop if they do not want to end on a cliff-hanger. However, I figure that they will have a cliff-hanger ending. Probably the first movie ends with the arrival of the Death-Eaters at the tent when they capture the trio.
On another website, it was agreed that the Silver Doe and Forest Again are the two main theological chapters in the final book.
Silver Doe" is about how to live a worthy life: be remorseful and face your flaws, destroy them (with the sword with its Cross-shaped hilt) and become a better person with a handy Christ figure (Harry) urging you on to discern truth from lies.
You have the sinner, Ron, then comforted by the Christ figure, Harry. Harry sits down, as a dear friend, tells Ron he is valued, tells him he was missed, tells him that he has redeemed himself, and brings him back to the shelter of the tent.
If Ron's return is delayed to the second movie, the theology of it may get divided too.
Therefore, in the first movie, Ron's Dumbledore gift of light is truly understood and used and so is Hermione's gift from Dumbledore, of wisdom from her book.
Harry's gift will be explored in the final movie. "The Forest Again" is about how to face death. We all have to take that walk into the forest at the end of our lives.