I've got the OLYMPIC plans (got them for my MA thesis), and the Marine landing beaches were at Kushikinio, west of Kagoshima City. The V Amphibious Corps was given the Kushikino mission, and that meant 2nd, 3rd, and 5th Marine Divisions. 2nd and 3rd MarDivs would have made the assault, with 5th in floating reserve. Their X-Day opponent was the 303rd ID, coastal-defense division raised in June '45. The beaches chosen for the Marines only had a reinforced battalion defending each beach, as the Japanese expected the landings a little further to the north.
After the surrender, VAC had the job of occupying Kyushu, and they found that the Japanese 40th Army (defending SW Kyushu) was very short of just about everything. Motor vehicles and fuel being at the top of the list. The Marine planners expected any counterattack to begin within 36 hours of the initial landings, then they found out from talking with their Japanese counterparts that it would've taken nine days. By that time, VAC has broken out of the beachheads and is pushing north to Sendai and east to Kagoshima City. Before they were demobilized, the 40th Army staff participated in a wargame with VAC, and both sides agreed as a result of the wargame, that though the terrain greatly favored the defender, the American advantages in firepower, the lack of fuel and motor vehicles to move reinforcements to the beaches, American air superiority, the very poor condition of roads, and with only one mobile division (77th ID) and a tank brigade meant that VAC would have accomplished its mission by X+30.