Add return vias in reference plane changes
There are many places where the signal changes between L1 (or L3) and L6 (or L8) through successive microvias, an easy way to spot those is to look at the vias on L4-L5 (first screenshot). In these reference plane changes the two GND planes (L2 and L7) need to be stitched very close to the transition to provide a clean return path for the signal current. In many places that's not an option because of the high PCB density (components on both sides) but there are some places where it does seem easy to add them, I'm listing some examples below (second screenshot). You can try to scan the board for such examples by looking at all the layers and trying to find "holes" where through vias can be added without rerouting.