Yes, without the side stories there are too many open questions.
I don't know if you wanted to describe the end without side stories or if where you read they didn't have the side stories. If the later is the case, there actually are three side stories (another one will be added after RAIYC or next year), two of them kind of continue the main story.
It was the same earlier this year with DMIYC, it felt much better after the author released extra 1, ~ two weeks after the main story ended. The ending of the main story was too sudden.
And Grayson can be a big asshole, but he isn't really a villain. He did cause trauma on Chase, but it wasn't intentional. He isn't even aware of it at that point. That doesn't make it any better for Chase, but that might be the reason why it felt unsatisfying for you that "Grayson was just there" and wasn't dealt with as a normal villain would be.

it was such a disappointment. It was interesting and all until everyone started getting shot & hospitalized or kidnapped and then Grayson was a villain that was just... there I guess. The secret pregnancy thing turned absolutelly irrelevant: the three of them (Chase, Josh and Pete) were at an amusement park and then suddenly were at some big abandoned place with wild dogs? And Chase had a big moment where he ignored the dogs and saved Pete and then Josh was like "it's your child too. And btw you never had unconscious sex with a dog Grayson was just mean and lied." and Chase basically ignores Pete and goes all about the dog's issue. Next scene is a huge timeskip to show they lived a happily ever after. And that's it. Such a lack of flavor.