The next phase of Brad Pitt and Angelina Jolie’s divorce — which involves their finances — could be just as tedious as their long custody battle, a source tells Us. The estranged couple made headway before the holidays when they reached an agreement about their six kids (Maddox, 17, Pax, 15, Zahara, 14, Shiloh, 12, and twins Knox and Vivienne, 10). But finalizing a permanent child support agreement has been “a slog” for the multimillionaires, says the source. For now, the World War Z star, 55, will continue to pay “substantial temporary child support,” which the source says is quarterly, not monthly, because Angelina is also such a high earner.…