Finished all the main endings of Zero Time Dilemma, it ends the series on a satisfying note.
Unlike VLR, there are no alternate solutions to puzzles in order to obtain documents needed for the plat so there isn't any need to replay puzzles or follow a guide to get the documents in the 1st playthrough of the puzzle. All you have to worry about is completing them. They seemed a little easier this time, it was only every 2 or 3 puzzles that I would get stuck on a step and need help to get past it to finish a puzzle. For instance, one gives you a screw driver and I was all over trying to find something to unscrew. Looked it up and it was this large panel that I thought was just a wall. The screws on it are so tiny you can barely see them on the vita screen. That tripped me up more than a couple of times, being unable to distinguish background objects from items that have a function as there wasn't a visual cue.
999 is connected it a few ways to VLR, but VLR and ZTD have many more connections. So I would definitely recommend playing at least VLR first and not jumping into ZTD cold.