I’d definitely combine 5/6 and 7/8 for reading, beginning with the 5th & 7th readers (unless students had those last year, in which case you could begin with 6 & 8.) This would give you opportunity to teach reading rather than “assigning” it. Adjusting assignments as needed to meet some varying capabilities in the class is not so big a challenge as is not being able to teach either class well.
Having publisher-made ancillary materials is helpful to use as the “default” in teaching a class. This provides reasonable work with each lesson while freeing you to introduce alternative assignments such as writing assignments which can often be assigned as a takeoff from some reading assignment.