Fixing Roof Sheathing Lacking Spacing
For your situation, I would cut the gaps like you suggest and add blocking to force the alignment of the edges. Plywood clips should have been used to keep the edges in alignment, but the gaps should have been there too. Over cut the gaps enough to make sure the butt joints are open totally

You will only need to use scraps to block the edges, you could even use 3/4" plywood for the blocking using 2 screws on either side of the gap. I would do the same with 2X if you used that as well, that way one screw will help the other draw the blocking tight to the plywood. The blocks do not need to go over the gap parallel to it, it can go perpendicular to the gap to bridge the two. That is how I described the screw install.