I was looking at the Wikipedia page for peleliu https://en.m.wikipedia.org/wiki/Battle_of_Peleliu and in the aftermath section it mentions how the Japanese made major improvements to their strategy. But that the Americans didn’t really change their plans much from earlier battles.
I know a large point of contention is the brutality of the pacific war and how at the time people were getting upset with the loss of life to capture these tiny islands.
Why was there so little American change of plan? I know they bypassed some islands that were heavily defended and this was a successful plan, but it seem that they could have preserved troops much better on later islands.
Additionally, were there innovations planned for dealing with the imminent invasion of japan.
My understanding here is that you're asking three questions.
My answer a few days ago here has some examples of how both sides changed their tactics in island warfare as the war progressed. While maybe not exactly what you're looking for, u/lubyak's answer and u/wotan_weevil's linked answer also provide some excellent context.
The Americans had poor intelligence at Peleliu that contributed to overconfidence and poor planning. Planners generally expected the Japanese to defend the beaches intensely (similar to Tarawa and Saipan), and to fall back to defend the airfield. After seizing the beaches and taking the airfield, it was assumed that cleaning up Japanese forces on the rest of the island would be relatively quick. It's not a large island -- only five or six square miles -- and the Americans had come to believe they understood how the Japanese would fight.
The Japanese did place mines and some defenses on the beaches and at the airfield, but instead chose to concentrate their main defensive positions in what came to be known as the Umurbrogal pocket. The defense of Peleliu was commanded by Col. Kunio Nakagawa, who chose this course where some other commanders in the Palau did not, or didn't have the terrain to do so (and were quickly wiped out as a result). The mountainous terrain in the Umurbrogal was treacherous; while only about 550 feet tall, it gave the Japanese better vantage points and natural defenses in addition to their own constructions of caves and tunnels. This also had the added benefit of creating interior lines between the Japanese positions that could be defended successively, rather than building a thin line of defenses around the beaches that could be broken and then flanked.
American reconnaissance was mostly conducted by submarine, which showed that the defenses on the beaches appeared to be manageable. In addition, there was little in the way of pre-invasion aerial bombardment, partially because the Americans hoped to capture the airfield relatively intact. There was a large invasion fleet to conduct a naval bombardment before the invasion, but that would not have focused on the Umurbrogal pocket. Fighting for the airfield was indeed intense, not the least because the Japanese artillery and mortars in the Umurbrogal were directed onto the airfield. The Americans decided not to invade the larger island of Babeldaob in the Palaus, which actually had more Japanese defenders than Peleliu. It was bombed mercilessly.
According to this paper, there were more than 600 prepared Japanese positions on Peleliu, mostly in the Umurbrogal area. That figure doesn't seem to include ad hoc foxholes or small positions used by just a few men; that's more than 600 caves, tunnels, bunkers that were carved out before the invasion. The largest of these could house hundreds of Japanese soldiers. The Americans responded with a buffet of tactics that had either been developed in earlier battles or were still being developed, using artillery, air support, tanks, flamethrowers, grenades, and even bulldozers to clear or seal off positions one by one. The defense of Peleliu also did not see any large-scale banzai attack near the end of the fighting. Instead, the Japanese remained in their caves until there were only a few dozen remaining. It was ugly, nasty fighting that took more than two months and chewed up the legendary 1st Marine Division.
Both sides learned from the experience. The Americans conducted significant aerial reconnaissance of Iwo Jima, for example, that showed the Japanese garrison there had been preparing underground defensive positions on the island. Iwo Jima suffered a massive air and naval bombardment before the invasion, but of course, it was still brutal combat. It's a fundamental truth of ground combat that it's difficult to eradicate an opponent who has well-prepared defensive positions when you're the attacker. The Americans got better at it, but it was never going to be easy.
This is probably a big enough topic for its own question. Early plans for the invasion of Kyushu projected that it would be carried out in November 1945, and I'm not sure how fleshed out the plans were by mid-August.