In order to get the "True" Ending, after satisfying the requirements you mentioned for the Good ending and reaching Chapter 9, before you complete the quests that are assigned to advance the story, there are a bunch of Events you have to see all across Gamindustri. These are all located in hidden dungeons (which is hinted at a few chapters earlier by a townsperson), however, so I hope you've been using the Scout System to find all of them. To find dungeons, you just need to send scouts to the right section of the world map, it doesn't matter which dungeon they search in.
The specific dungeons are as follows, there are six events you want to find:
- Leggo Island (Planeptune)
- Vita Dimension (Lastation)
- So Shal Forest (Hello Island)
- Keraga Dimension (Hello Island)
- Pi Shii Game Factory (PC Island)
- Graphic Pass (Eden)
That last one, a lot of people miss. Don't forget to scout Eden in Chapter 8, it'll make it easier on you. Oh, also, if you have been going at a normal pace, not been overly leveled by doing all the quests, don't fight enemies in any of these dungeons besides Graphic Pass. You will be mercilessly slaughtered by enemies doing 8k damage or higher. Even Penguins.
The events do not have any specific order that you need to attend to them - in fact, they will always play in the correct order no matter what dungeons you visit in which order.
Once you've seen all the events, then you shall have qualified for the True ending! Make sure to still attend every other event that comes up as you go through completing Chapter 9. You'll know you're definitely on the right track when the following happens shortly into the start of Chapter 10.
When Histoire opens the portal to travel between the dimensions, it will actually be giant and will allow free travel between the two dimensions of Gamindustri. Which is as awesome as it sounds, too.