@Teddy-Stonehill I haven't heard of the Libri app and after reading the first couple of reviews, I'd stay away from it too. It also looks like another pop-romance book site. If that's not your genre, it's probably not worth it. I generally operate on the idea that if a platform is reputitable, they'll contact you asking for your reps and/OR there will be plenty of information about them. I'm not a fan of Radish but I know a lot of writers who've made decent income from it. Radish has a very clear application process for its new readers.
In regard to Amazon and Kindle Vella - wait and see. I'm still excited for the platform but if you want to explore serialized fiction and your work isn't pop-romance, I'd wait until there is more data to play around with. There is still a lot up in the air there.