For me, being in a fandom means I'm talking in it or contributing to the discussions. So stuff that I'm into but just aren't interested enough to talk about publicly wouldn't count.
That being said...
Pokemon, I've gotten into chatrooms when I was younger and one forum in my teenage years and now this one, and I've followed it all super actively.
RWBY, surprisingly, while I was in high school from Volume 4 to Volume 8, a lot of discussion and a few off-topic forum threads. I don't really consider myself part of the fandom anymore though.
BanG Dream, I got into it maybe 4 or 5 years ago when the game was still new, played and posted about it daily and interacted a lot with its Instagram community.
Love Live, of course, which I've been into for probably 4 years. I interacted with both the Instagram and Twitter communities for it, wrote blog articles on it, shared that with several people on those two social media platforms along with Reddit, followed a couple streamers, watched most of the Aqours lives, and actively played SIFAS daily for most of its lifetime.
New Game, which I got into before the previous two but didn't really start interacting with the community by openly discussing it and joining the discord server and being there for a while.
FFXIV, I'll contribute to discussions, watch things like the live letters, play the new updates the day they release, participate in the seasonal events, and participated in the server test they ran a few weeks ago.
And Machikado Mazoku, which I got into just a couple months ago but I'm already in love with it and discussing the manga and how eager I am for any crumb of an update I can get my hands on T^T