Whenever you’re confronted by a tedious task in After Effects – such as splitting a single layer into a grid of smaller images – I highly recommend checking whether there is already a script that can help you. In this particular case there are two.
Slicer, from Redefinery, has a little UI of its own that lets you decide how to slice and dice, and whether to parent all the new layers to a null. It even offers to set everything to Alpha Add mode so that the overlapping borders don’t cause gaps.
Split Image is included with the NABscripts collection. It has less options – just the number of rows and columns – but it offers one major difference from Slicer. Each slice is placed into a pre-comp which is just the size of the slice – that way if you want to cross fade the grid into individual clips, you’re all set.