I’m not sure expressions will be able to solve this.
The way I would approach it would be to pre-comp all six sides of the cube separately. You then do the fly-in animation in each of those pre-comps.
From there – put all those cubes together in the main comp, and make sure each pre-comp has the * Collapse Transformations / Continuously Rasterize button is selected.
To start out, just build one side of the cube and use it 6 times, then if you need each side to be unique, just swap them out.