Yes, you can revisit the islands and you will not miss anything important.
There will be certain places (caves, buildings) that are only accessible in missions while you are doing the specific mission. Once the mission is over, you might not be able to access the specific place anymore.
This will include items that you can gather in specific locations. If you want to gather the maximum amount of items, it is recommended that you pickup everything that is not accessible as an open space (i.e. inside caves, houses, fenced in areas) during the mission. Everything that is visible as loot on the large map should be accessible anytime.
In other words:
Once you finished all missions, you can fast-travel to both islands and access all relics and loot that is on the main map visible by buying the maps from the shop. However, if a mission brought you inside a house where you could pickup something, it might be unavailable once that mission is finished. Those items normally do not show up on the large world map however.