Oki was an 82 day battle, Iwo was 35 days. Oki had a monster sea battle against the kamikazes, Iwo didn’t. At Iwo, the American casualties surpassed the number of the enemy forces. Iwo Jima US casualties were 26000. As for a the significant step up in casualties, the naval forces suffered more casualties than the ground forces, definitely not the case at Iwo Jima.
Naval engagement was necessary in order to establish the attack on Okinawa. Because of it, it made bomber attacks on the Home Islands possible. There was no way they could get around that engagement.