The achievement description does a poor job informing players exactly what "all events" entails. Your goal is to achieve 100% completion on each planet in the galaxy map, and you will unlock this the moment you finish your last "Completed all events on..." achievement. (For example, mine unlocked as soon as I finished the Western Reaches achievement.)
For clarity, this means your only concern is earning all exploration kyber bricks (bricks out in the open), datacards, characters, and starships in every planet. This achievement does not require completion of your Missions tab (minikits, level challenges, side missions, puzzles, trials, and challenges), nor does it require completion of any capital ships.
If you've gotten this far in the game, it's likely you're going to finish your Missions tab anyways; you'll need to for several other achievements. This solution simply offers a more accurate explanation of the unlock conditions.