Like Vris said, waiting to buy Splatoon 3 instead is the better move at this point, since we know it'll be coming out next year and most people will be playing it instead of Splatoon 2 once it's out.
definitely suggest waiting for splatoon 3. at this point, i don't think spla2n is worth it to play unless you'd be also snagging the octo expansion (and i can't exactly vouch for doing that since it's an extra 20$). if you enjoyed the gameplay during the demo, but not A Ton, then you'd be better off just not picking it up and keeping an eye on splat3.
I'd get it if you can afford it. It's still rather "active", more so a lot of trolls and smurfs around but if you have someone to play with or just like it then get it. I mean it's not 2022 yet and it's definitely one of the most fun games and times I had on Switch.
As for the expansion, personally I didn't find that especially worthy cause I never bothered to complete it.