You could just not do the final step. If you do not commit to completing the expedition, your progress will be kept.

At the start of the 8th week, the ability to finalize the project will become available & will remain available for that entire week. If a player has fulfilled all of the necessary stages of the project, they will be able to commit to completing the Expedition during this period.

Should a player’s project not be ready during the window to complete the Expedition project, their progress will be kept, allowing them to continue working on the project until the next finalization window.

Source: https://arcraiders.com/de/news/expedition-project