The quest actually counts how many UNactivated monoliths are left, not how many monoliths you actually activated - so activating a monolith before the quest isn't a problem. If it says they aren't all activated, there's still an unactivated monolith on the depths. Note, the monoliths on other planets do not count.

thank

might be considered a spoiler but you can build a submarine that resists depth damage to explore further down. hopefully this is enough information to help you find it without spoiling but happy to give more info if needed