Splatoon 2 is fine. It's a fun game. Running, gunning, inking, Salmon Run, it's all very fun. The main campaign for Splatoon 2 was... mediocre and short.
Octo Expansion, however, blew my brain out of the back of my skull. It is packed with absolute JAMS, it's fun, it's got an amazing sense of progression, a terrific upgrade system, amazing characters, MORE JAMS, and a truly fucking MAGNIFICENT final scene that had me crying. I didn't play until after Splat 2 had nearly shut down (in preparation for Splatoon 3) so I never got to know Marina and Pearl through their Splatfests and understand how deeply connected they were. Octo Expansion still made me sob. Fucking incredible. Play it whether you like Splatoon 2 or not. Play it after Splatoon 2 goes offline. Play it until the sun burns out. It really is that good.