You seem to be asking two separate questions here, of which I can answer one.

1. Were the soldiers stationed on Iwo Jima all preparing for their first time fighting personally against American troops?

I don't know about the composition of Japanese forces on Iwo Jima at the time the Americans invaded.

2. Did the Japanese evacuate island garrisons when it became clear they were losing battles in the Pacific? ("Were there cases of Japanese soldiers that were pulled back to prepare for the next battle or was it fresh troops each time on their side going up against battle hardened allies?")

Mostly no.

There are limited examples of large-scale evacuations. The most prominent when the Japanese withdrew about 10,000 troops from Guadalcanal in early 1943. The Guadalcanal campaign was bitter attritional combat on land, sea, and in the air, made worse by tropical diseases and the struggle to keep troops on Guadalcanal supplied. Both sides poured men, ships, and aircraft into the region and suffered significant losses, but the Japanese especially struggled to deliver supplies to Guadalcanal. When it became clear the Japanese could not support an offensive there, the decision was made to withdraw as many troops as possible. The Japanese landed a small contingent of fresh troops to cover the evacuation, and over the course of a little over a week, were able to get just over 10,000 men, the remnants of the 2nd Infantry Division and 38th Infantry Division, off the island. Wounded and sick men were left, as were the bodies of those already deceased.

Another notable evacuation occurred at Kiska Island in the Aleutians in 1943. The Japanese had captured Kiska and other islands in the Aleutians in June 1942, but the Americans retook the island of Attu (west of Kiska) in May 1943. There were plans to reinforce Kiska before the Americans invaded, but that idea was dismissed in favor of withdrawing the troops on the island. A group of Japanese destroyers and a couple cruisers made several failed attempts to reach Kiska in July 1943 before eventually slipping through the American navy forces thanks to thick fog and some faulty radar readings. More than 5,000 Japanese were taken off the island, and the Americans were so unaware thanks in part to the fog that the Americans proceeded with their invasion plans in August. They found an empty island and some booby traps.

However, during the later campaigns through Pacific islands (like Iwo Jima and what you're probably thinking of), there was no way for Japan to withdraw its garrisons and thus no effort to evacuate them. Units were expected to fight until they were wiped out or ran out of supplies, in which event they generally committed suicide or executed banzai charges at the American lines. Mostly, entire garrisons were wiped out. Small numbers surrendered. A few unorganized stragglers continued to conduct guerilla warfare in some cases. The Americans did not invade every island, and other garrisons of Japanese troops were simply stranded beyond the reach of the Japanese navy to supply them.

By late 1943 (if not before) Japan was unable to challenge American naval supremacy, which brought with it hundreds of carrier-borne aircraft and air supremacy. Any attempt to rescue men from these islands would have meant the destruction of the ships sent to do so, and shipping was needed to keep up supply routes where Japan could still deliver goods. The last large-scale attempt by Japan to challenge the US fleet came during the invasion of the Marianas, which resulted in the Battle of the Philippine Sea (also called the "Marianas Turkey Shoot"). However, this still wasn't an attempt to withdraw troops from the islands there. The scattered remains of the Japanese navy had another go at the Americans at the Battle of Leyte Gulf, but again, the ships weren't intending to cover a withdrawal of troops from the Philippines. Japanese units garrisoned on islands late in the war understood they would not be relieved or withdrawn and fought accordingly.