1. No romex. Not rated for underground.
Use individual conductors. Black, black and white #8 and green #10 ground.
2. 2.5" conduit is way overkilled. 1" is plenty.
3. Neutral bus (white wires) is to be isolated from the enclosere. Do not install bond screw. Terminate grounds on a ground bar kit.
Also, install two 1/2" x 8' ground rods at least 6' apart and run a #6 wire to the ground bus.