Ok hear me out, I know we all love talking about how the time period of Grian getting possessed is taking less and less time as things go on, but what if there was a reason for that?
The Watchers did not like Grian's affinity for mischief and rebellion, they didn't realize when they made him one of them that it was inevitable he would rebel but they quickly realized. So, they carved an empty space in his mind, making him oh so easy to possess if he got too out of hand. However, despite this countermeasure, he escaped.
The trauma runs deep, and Grian is secretly constantly scared the Watchers will find him and use that space to possess him and turn on his friends. So he comes to the reasonable conclusion (in his mind) that if he doesn't want the Watchers in that space then he just needs to fill it!
The Mycelium was never malicious and actually quite liked the greenery, however she's never gotten to interact and play with the players before! Grian's deal with her means she gets to play pranks and have fun along with everyone else! She doesn't mind sharing control over Grian's body so long as she gets to be apart of the fun, and had no issue leaving Grian when they won the war because she had so much fun!
The Void was lonely more than anything, and they rarely got so much attention, and the sacrifices were really nice of boatem to provide. (Though the Void was far more interested in Scar than Grian, Scar's mental shields were too great to even think about possessing him, plus Grian gave Scar so much attention the Void hardly had to do anything anyways)
The moon didn't really possess Grian, just occasionally came along riding in Grian's head in the possession equivalent of carpooling. The moon just really liked seeing the man who made a whole cult in their honor through the eyes of someone who loves him. The moon really just sticks around to see the silly little cult man and their favorite moth through the eyes of someone else. (The moon has a long established deal with Pearl but really likes the new perspectives sometimes)
The Entity was more of a kid than anything, and only stayed with Grian long enough to give him instructions on how it wanted to look, and quickly moved to possessing its new body. It took a bit of time for the Entity to learn that players were kind of fragile and squishy unlike Grian, but it quickly learned, and was very happy to finally be able to communicate on a level everyone could hear instead of the frequencies only something as powerful as a Watcher could hear.
The Rift was far more cautious than its younger sibling, but when it saw how Grian treated the Entity, even when the Entity made mistakes, the Rift reached out. While the Rift was far older than the Entity, it was not unlike a touched-starved young adult, old enough to understand things better, but not yet fully matured, and quietly desperate for any contact. (The Rift was Grian's favorite, though he would never admit it, and he felt a lot of kinship to it) The portal to Empires was the Rift's attempt at a thank you, giving Grian an opportunity to see Jimmy again.
The Skulk tried to possess Grian, it was probably going to take advantage to how vulnerable Grian was to possession, however it was a bit to spread out (ha) to give a good effort to overcome Grian and the Rift was too powerful to overcome its stake on Grian. It never got further than one floating bolder.
The Ocean was not unlike Mycelium in just wanting some mischief and the Void in wanting attention. However she quickly realized she was far more interested in the attention the pretty red-head gave her. She was having fun with Grian, but she realized she felt rather bad (she's never felt guilt before and she really doesnt like it) upon messing with Grian by giving Scar the mending book. She quickly decided to fix her mistakes, and gave Grian a mending book of his own, however she does occasionally come over when she is in need of a friend and Grian wants a break (excuse) from the permit office.