On the positive side, verbally-gifted children generally find their own intellectual stimulation, in the form of voracious reading (and often writing).
The range of composite scores obtained for her cognition is actually fairly consistent (even though some fall to one side of the arbitrary cutoff, and others fall to the other side). The Superior range on the WPPSI is only a couple of points higher than her WISC-V FR and WM scores (potentially well within the confidence interval). Based on her WISC-V profile, her WIAT-III scores are consistent with predicted scores.
Note that the WISC & WIAT have linkage studies, while the WISC and WJ do not. Also, do you mean the WJ/WIAT difference is in percentage points, or standard score points? 10 %age points above a 110 isn't really significantly different, when you are comparing different tests. On another note, the reading comprehension and written expression tasks on the WIAT and WJ are quite different, with both tasks generally more challenging and authentic on the WIAT than on the WJ. RC on the WJ can be answered with single words, is fairly concrete, and uses shorter passages, whereas the WIAT RC includes both literal and inferential comprehension, and is in the context of lengthier pieces of grade-level text. The Sentence Combining portion of the WIAT, in particular, requires more complex written language skills than many of the WJ writing tasks, which I would say are more like the Sentence Building portion.
But more important than the exact scores is the behavior that you are reporting. Whether she tips the official score minimum or not, you are describing a bright, talented young person, with a thirst for learning that is starting to dry up. You have plenty of documentation that she has verbal gifts.
In the past, some of my DC's teachers have offered alternative reading selections, or suggested higher-level reading just for "reader's workshop" or "book club" periodic check-ins with the teacher. No written assignment, just sporadic, intentional conversations about these individualized reading selections, and a running list of titles read. It didn't require the teacher to make any major changes to her lesson plans, just a five or ten-minute chat (with an official name!) every week or two. Somehow, I think having a little bit of something to look forward to makes the other classwork just that much more bearable.
